Label Free Sensing

Optical Fibre Gratings

Optical fiber sensors were seeing a groundbreaking development in the last twenty years, not only in both the fundamental and applied researches but also in the industry.

Thanks to the peculiarities of optical fibers, optical fiber gratings (LPG and FBG) have recently been suggested as alternative optical platforms for chemical and biochemical label-free sensing, on the basis of the measurement of refractive index changes induced by an interaction.

The group also collaborates with:  

• The Department of Electrical Engineering of the University of Pisa (G.Barillaro), who is leading the research finalized to the development of optical sensors based on photonic crystals; 

• The Department of Industrial and Information Engineering of the Second University of Naples (L. Zeni), who has led the research activity for the development of SPR-based optical sensors Within this collaboration, a thermo-stabilized flow cell was designed and implemented for the integration of plastic optical fiber (POF) SPR sensors. 

Projects

BOHEMIAN (“Versatile hybrid in-fiBer Optical-electrocHemical systEMs for wIdely Applicable biosensing”)

Bilateral Project S&T Programme (Joint Research Proposal under CNR, Italy – CSIR, India) 

Optical Resonators

Label–free optical biosensors based on Whispering Gallery Mode (WGM) microresonators have been proposed as special optical platforms capable to reach single molecule detection, with the first example given by biosensors implemented on glass microspheres carrying on their surface the selective sensing layer. Particular attention is devoted to the design, development and charaterisation of microbubble resonators constituted by spherical bulges realized in hollow silica microcapillaries.

They present the advantage of incorporating in the same device the exclusive properties of WGM resonators (i.e.: high Q factors – typically > 107 in air – and small mode volumes), with embedded microfluidics ensured by the microcapillary itself. 

Projects

SAFEWATER (“On chip whispering gallery mode optical microcavities for emerging microcontaminant determination in water”)

SENS4BIO (“Microrisonatori Optofluidici Ultra-Sensibili di Tipo “Flow-Through” per Applicazioni Biosensoristiche”) 

Lossy Mode Resonance

Lossy mode resonances (LMRs) arise in the transmission spectrum of a cladding-removed multimode optical fiber coated with a thin film. The sensing performance of such devices, particularly their sensitivity to changes in the coating characteristics or the surrounding medium, can be optimized by suitably tuning the coating refractive index, film thickness, and surrounding medium refractive index. By immoblising selective biological recognition element, it is possible to develop highly sensitive optical fibre chemical and biochemical sensors.

Projects

FOCAL (“Fiber Optics sensors as a platform for CAncer diagnosis and in vitro modeL testing”)

Main Publications on the subject

G Frigenti, N Marcucci, F Baldini, C Trono, Multiplexed detection using two serial long period gratings in a Mach-Zehnder interferometer configuration, Optics & Laser Technology, 193, Part A, 114129, 2026

Abstract: We report a novel methodology for multiplexed detection in optical fiber sensors by employing two serial long-period fiber gratings (LPGs) arranged in a Mach-Zehnder interferometer-like configuration on the same fiber. This configuration enables simultaneous measurement of multiple parameters by analyzing the interference pattern resulting from the interaction of the LPGs with their surrounding environment. By tracking selected spectral features of the interference fringes and modelling their evolution with simple bivariate polynomial functions, it is possible to infer the temperatures of the two LPGs, which were used as measurand of interest for our proof-of-principle experiment. Experimental results demonstrate high precision (in the order of the instrumental error) and resolution comparable to that of single LPG-based sensors (40 pm, which is 5× 10− 4 times the full-width at half-maximum of the LPG peaks). Furthermore, the proposed method is mathematically general and can be easily extended to other types of measurands (eg refractive index, strain) and to the analysis of the spectral characteristics of other interferometric systems. In perspective, it offers a pathway towards accurate and robust multi-parameter detection of any measurand or analyte to which LPGs are sensitive, with potential impact in biosensing and other applications where multi-parameter tracking is essential.

F Pakfetrat, M Bahadoran, S Dabagh, F Chiavaioli, Design and modeling of eye-shaped double-slot GaAs microring resonator for CO2 sensing, Scientific Reports, 15(1), 31822, 2025

Abstract: Monitoring of CO₂ is crucial because of its profound impact on both environmental and human health. A novel highly sensitive refractive index (RI) sensor, utilizing a double-slot microring resonating structure, has been designed and numerically assessed for the sensitive detection of gas media. The structure consisted of a circular microring resonator nested in a racetrack resonating configuration mimicking the structure of an eye-shaped microring resonator (ESMRR). This system was simulated and designed for a GaAs double-slot core waveguide deposited on a suitable Al0.5Ga0.5As substrate. Optical transfer function of ESMRR and related equations were derived using Mason rule, while the numerical analysis was performed using the variational finite difference time domain (var- FDTD) method. The free spectral range (FSR) was extended to 137.68 nm that led to a remarkable bulk sensitivity of 1217.39 nm/RIU and a resolution of 4.93 × 10−6 RIU. The proposed sensing structure was envisioned for CO2 sensing and demonstrated an impressive sensitivity of 24.4 pm/ppm for CO2 detection with an estimated detection limit of 0.82 ppm.

D Santano, AB Socorro, A Giannetti, I Del Villar, F Chiavaioli, Simultaneous detection of inflammatory process indicators via operando dual lossy mode resonance-based biosensor, Opto-Electronic Science, 4 (10), 250005, 2025.

Abstract: Detecting multiple analytes simultaneously, crucial in disease diagnosis and treatment prognosis, remains challenging. While planar sensing platforms demonstrate this capability, optical fiber sensors still lag behind. An operando dual lossy mode resonance (LMR) biosensor fabricated on a D-shaped single-mode fiber (SMF) is proposed for quantification of clinical indicators of inflammatory process, like in COVID-19 infection. Dual LMRs, created via two-step deposition process, yield a nanostructure with distinct SnO2 thicknesses on the flat surface of the fiber. Theoretical and experimental analyses confirm its feasibility, showing a sensitivity around 4500 nm/RIU for both LMRs. A novel insight in spatially-separated biofunctionalization of the sensitive fiber regions is validated through fluorescence assays, showcasing selectivity for different immunoglobulins. Real-time and label-free detection of two inflammatory markers, C-reactive protein and Ddimer, empowers the platform capability with a minimum detectable concentration below 1 μg/mL for both biomolecules, which is of clinical interest. This proof-of-concept work provides an important leap in fiber-based biosensing for effective and reliable multi-analyte detection, presenting a novel, compact and multi-functional analytical tool.

S Soares, A Giannetti, F Esposito, L Sansone, A Srivastava, S Campopiano, M Giordano, M Facao, NF Santos, A Iadicicco, C Marques, F Chiavaioli, Cortisol detection using a long period fiber grating immunosensor coated with graphene oxide, Sensors and Actuators Reports, 9, 100279, 2025.

Abstract: Recirculating Aquaculture Systems (RAS) have revolutionized the protein production sector in aquaculture, leading to significant growth and expansion of the industry. Despite the success of RAS in aquaculture, there are challenges related to stress in fish raised in these systems, which can impact their food intake, growth, and overall well-being. One of the major limitations in the aquaculture industry is the lack of smart sensors for real-time detection of stress hormones like cortisol, hindering our ability to understand and effectively manage the welfare of fish in these systems. In this work, a graphene oxide (GO) coated long period grating (LPG) was fabricated into a double-clad optical fiber (DCF) with W-shaped refractive index profile. The working point of the device was tuned to the mode transition region to enhance its sensitivity against outer medium changes. It was further integrated into a microfluidic system and the fiber surface was functionalized with specific anti-cortisol antibodies for the detection of cortisol. Finally, the performance of this immunosensor was evaluated for a cortisol concentration range of 0.01 ng/mL to 100 ng/mL, a wide working range of concentrations of relevant interest, achieving a limit of detection (LOD) of 0.06 ng/mL. Moreover, a selectivity test using testosterone and glucose as interfering substances was carried out.

M Vrabcová, M Spasovová, M Forinová, A Giannetti, M Houska, NS Lynn, F Baldini, J Kopeček, F Chiavaioli, H Vaisocherová-Lísalová, Optical fibre long-period grating sensors modified with antifouling bio-functional nano-brushes, Biomater. Sci., 13, 1199-1208, 2025

Abstract: Recent advances in optical sensing technologies underpin the development of high-performance, surface-sensitive analytical tools capable of reliable and precise detection of molecular targets in complex biological media in non-laboratory settings. Optical fibre sensors guide light to and from a region of interest, enabling sensitive measurements of localized environments. This positions optical fibre sensors as a highly promising technology for a wide range of biochemical and healthcare applications. However, their performance in real-world biological media is often limited by the absence of robust post-modification strategies that provide both high biorecognition and antifouling capabilities. In this study, we present the proof-of-concept antifouling and biorecognition performance of a polymer brush nano-coating synthesized at the sensing region of optical fibre long-period grating (LPG) sensors. Using a newly developed antifouling terpolymer brush (ATB) composed of carboxybetaine methacrylamide, sulfobetaine methacrylamide, and N-(2-hydroxypropyl)methacrylamide, we achieve state-of-the-art antifouling properties. The successful on-fibre ATB synthesis is confirmed through scanning electron microscopy (SEM), fluorescence microscopy, and label-free bio-detection experiments based on antibody-functionalized ATB-coated LPG optical fibres. Despite the challenges in handling optical fibres during polymerization, the resulting nano-coating retains its remarkable antifouling properties upon exposure to blood plasma and enables biorecognition element functionalization. These capabilities are demonstrated through the detection of IgG in buffer and diluted blood plasma using anti-IgG-functionalized ATB-coated sensing regions of LPG fibres in both label-based (fluorescence) and label-free real-time detection experiments. The results show the potential of ATB-coated LPG fibres for use in analytical biosensing applications.

VA Amorim, G Frigenti, F Baldini, S Berneschi, D Farnesi, PAS Jorge, JM Maia, G Nunzi Conti, PSS Dos Santos, P V S Marques, Integrated all-in-silica optofluidic platform based on microbubble resonator and femtosecond laser written surface waveguide, IEEE Sensors Journal, 24 (16), 25573-2558, 2024

Abstract: Optical microbubble resonators (OMBRs)–understood as localized thin wall bulges induced in silica microcapillaries-are gaining an ever-growing interest in microfluidic sensing applications due to their capability to sustain whispering gallery modes (WGMs) and confine the fluidic sample within their own hollow-core microcavity. Currently, most applications use an external tapered optical fiber for coupling light to the resonator. This arrangement is known to be fragile and prone to vibrations. In this work, an alternative approach, based on coupling OMBR with a femtosecond (fs) laser-written optical waveguides, integrated at the surface of fused silica substrate, is proposed. In this configuration, a stable and robust final structure is accomplished by gluing the two ends of the microcapillary, on which the OMBR is made, to the substrate. The OMBR quality factors, measured at the excitation wavelength of 1540 nm, show values close to 10⁴ in the case of a water-filled cavity, with a maximum coupling efficiency of up to 6.5%. Finally, the operation of the integrated optical devices as refractometers is demonstrated by delivering different solutions with successively increasing concentrations of NaCl inside the OMBR. An average sensitivity of 45 nm/RIU is obtained, yielding a resolution of 4.4 X 10^-5 RIU, creating the potential for this platform to be applied in chemical/biochemical sensing.

L Zu, X Wang, P Liu, J Xie, X Zhang, W Liu, Z Li, S Zhang, K Li, A Giannetti, W Bi, F Chiavaioli, L Shi, T Guo, Ultrasensitive and multiple biomarker discrimination for Alzheimer's disease via plasmonic & microfluidic sensing technologies, Advanced Science, 11(24), 2308783, 2024

Abstract: As the population ages, the worldwide prevalence of Alzheimer's disease (AD) as the most common dementia in the elderly is increasing dramatically. However, a long-term challenge is to achieve rapid and accurate early diagnosis of AD by detecting hallmarks such as amyloid beta (Aβ42). Here, a multi-channel microfluidic-based plasmonic fiber-optic biosensing platform is established for simultaneous detection and differentiation of multiple AD biomarkers. The platform is based on a gold-coated, highly-tilted fiber Bragg grating (TFBG) and a custom-developed microfluidics. TFBG excites a high-density, narrow-cladding-mode spectral comb that overlaps with the broad absorption of surface plasmons for high-precision interrogation, enabling ultrasensitive monitoring of analytes. In situ detection and in-parallel discrimination of different forms of Aβ42 in cerebrospinal fluid (CSF) are successfully demonstrated with a detection of limit in the range of ≈30–170 pg mL−1, which is one order of magnitude below the clinical cut-off level in AD onset, providing high detection sensitivity for early diagnosis of AD. The integration of the TFBG sensor with multi-channel microfluidics enables simultaneous detection of multiple biomarkers using sub-µL sample volumes, as well as combining initial binding rate and real-time response time to differentiate between multiple biomarkers in terms of binding kinetics. With the advantages of multi-parameter, low consumption, and highly sensitive detection, the sensor represents an urgently needed potentials for large-scale diagnosis of diseases at early stage.

S Cao, R Chen, Q Yang, X He, F Chiavaioli, Y Ran, BO Guan, Point-of-care diagnosis of pre-eclampsia based on microfiber Bragg grating biosensor, Biosensors and Bioelectronics, 249, 116014, 2024

Abstract: Pre-eclampsia is a serious multi-organ complication that severely threatens the safety of pregnant women and infants. To accurate and timely diagnose pre-eclampsia, point-of-care (POC) biosensing of the specific biomarkers is urgently required. However, one of the key biomarkers of pre-eclampsia, placental growth factor (PlGF), has a reduced level of expression in patients, which challenges the quantification capability and Limit-of-detection (LOD) of biosensors. Herein, we reported a microfiber Bragg grating biosensor for the quantification of PlGF in clinical serum samples. The Bragg grating was inscribed in a unilateral tapered fiber to generate the segmented Fabry-Perot spectrum for improving the capability of detection. Furthermore, a temperature-calibrated Bragg grating was added to enable dual parametric detection of PlGF and temperature simultaneously for removing the crosstalk. Finally, the biosensor was envisaged to be perfectly compatible with microfluidic chips, and thus dramatically reducing the sample consumption to as small as 10 μL. The proposed biosensor can respond to PlGF with concentrations ranging from 5 to 120 pg mL⁻¹, attaining a LOD of 5 pg mL⁻¹ of clinical relevance. More importantly, the biosensor achieved micro volume detection of clinical serum samples from patients, and the ROC curve with an AUC of 0.977 confirmed the viability of the device. Our study paves the way to a new idea for cost-effective and high-precision screening of patients with pre-eclampsia, and hence envisages a promising prospect for point-of-care (POC) diagnosis of patients with pre-eclampsia.

T Zhu, K Chah, F Chiavaioli, J Villatoro, C Caucheteur, Gold-coated optical fiber supermode interferometer for insulin bio-sensing, Optics & Laser Technology, 168, 109878, 2024.

Abstract: In this work, we present a supermode interferometer for biosensing application. The interferometer was fabricated with a segment of a seven-core fiber fusion-spliced to a standard single-mode fiber. We have found that when a thin gold layer was deposited on the multicore fiber (MCF) end facet, the response of the interferometer to refractive index (RI) changes was polarization dependent. The highest sensitivity to RI was achieved for a ∼10 nm-thick gold coated sample when P-polarized light was used. The gold-coated sample was later bio-functionalized and then used for insulin detection. The sensor was able to detect insulin with a concentration level of 10⁻⁸ g/ml.

TK Dey, C Trono, P Biswas, A Giannetti, N Basumallick, F Baldini, S Bandyopadhyay, S Tombelli, Biosensing by Polymer-Coated Etched Long-Period Fiber Gratings Working near Mode Transition and Turn-around Point, Biosensors, 13 (7), 731, 2023

Abstract: A methodology to enhance the sensitivity of long-period fiber gratings (LPFGs) based on the combination of three different enhancement approaches is presented; the methods here adopted are the working near mode transition (MT) of a cladding mode (CM), working near the turn-around point of a CM and the enhancement of the evanescent field of CMs by reducing the cladding diameter or by increasing the order number of CMs. In order to combine these enhancement methodologies, an electrostatic self-assembly (ESA) process was used to deposit a polymeric overlay, with a chosen thickness, onto the etched fiber. The add-layer sensitivity of the sensor was theoretically calculated, and the demonstration of the real applicability of the developed LPFG as a biosensor was performed by means of an IgG/anti-IgG immunoassay in human serum in a thermostated microfluidic system. The limits of detection (LODs) calculated by following different procedures (three times the standard deviation of the blank and the mean value of the residuals) were 6.9 × 10⁻⁸ µg/mL and 4.5 × 10⁻⁶ µg/mL, respectively. The calculated LODs demonstrate the effectiveness of the applied methodology for sensitivity enhancement.

TK  Dey, S Tombelli, A Roy, P Biswas, A Giannetti, N Basumallick, F Baldini  S Bandyopadhyay, C Trono, Sensitivity analysis of sidelobes of the lowest order cladding mode of long period fiber gratings at turn around point, Sensors, 22, 2965, 2022

Abstract: A new methodology to enhance the sensitivity of a long period fiber grating sensor (LPFG) at the Turn Around Point (TAP) is here presented. The LPFG sensor has been fabricated by etching the fiber up to 20.4 µm, until the sidelobes of dispersed LP0,2 cladding mode appeared near TAP in aqueous medium. The dual peak sensitivity of the sidelobes was found to be 16,044 nm/SRIU (surrounding refractive index units) in the RI range from 1.333 to 1.3335.

Z Li, X Yang, F Wang, H Zhu, X Jin, Y Duan, F Chiavaioli, Discriminating bulk and surface refractive index changes with fiber-tip leaky mode resonance, Journal of Lightwave Technology, 41(13), 4341-4351, 2022

Abstract: Optical surface wave excited by nanocoatings has enabled fiber-optic sensors with high sensitivity to the change in external environment generally characterized by bulk refractive index (RI). Nonetheless, this sensitive response inevitably suffers from the contamination of surface-localized binding events. Herein, we proposed and demonstrated a miniaturized fiber-tip leaky mode resonance (L^eMR) sensor capable of unambiguously discriminating bulk and surface RI changes from a theoretical perspective. The fiber-tip L^eMR sensor consists of a multi-mode fiber tip coated with TiO₂ thin film on which a surface-localized binding layer thinner than was set equivalent to the change of surface RI. The operation principle was obtained, with the analyses revealing that several fiber-tip leaky modes can be excited with fiber core mode and flexibly tuned by simply altering the thickness of TiO₂ thin film, which hence generates non-polarized fiber-tip L^eMR identified by the signature of attenuation maxima in reflection spectrum of the fiber-tip probe. These fiber-tip L^eMRs present a sensitive linear intensity response (and wavelength shift) to the change in bulk RI (and surface RI), which is considerably feasible for distinctly discriminating bulk and surface RI changes. The results highlight that both the highest bulk sensitivity up to 58.5 dB/RIU in the RI range of 1.315 ~ 1.355 and the highest surface sensitivity up to 0.595 nm/nm, combined with the smallest cross interference between them, can be attained simultaneously with a single fiber-tip L^eMR and without the need of reference channels, which could enable the development of ultra-compact lab-on-tip optrodes, a pathway of the lab-on-fiber roadmap.

G Moro, F Chiavaioli, S Liberi, P Zubiate, I Del Villar, A Angelini, K De Wael, F Baldini, LM Moretto, A Giannetti, Nanocoated fiber label-free biosensing for perfluorooctanoic acid detection by lossy mode resonance, Results in Optics, 5 100123, 2021

Abstract: The determination of per- and polyfluoroalkyl substances (PFAS) in environmental samples, such as drinking waters, requires the design of high performing and versatile sensing strategies. Label-free biosensing platforms based on specialty fiber optics are a valid option to face this challenge. Among them, lossy mode resonance (LMR) fiber optic biosensors are showing remarkable performance in terms of detection limit, selectivity, and reproducibility. The detection of small molecules, such as perfluorooctanoic acid (PFOA), can be achieved with the help of well-designed biological recognition layers. In this study, the biosensing potentialities of a label-free LMR-assisted optical platform based on nanocoated fibers are investigated. Delipidated human serum albumin (hSA) was used as biological recognition layer for PFOA in aqueous solution. Different fiber functionalization protocols based on the covalent immobilization of hSA were tested. The conformational changes related to the formation of hSA/PFOA complex were followed via optical monitoring of LMR spectral shift, showing a trend that can be modeled with Langmuir adsorption isotherm. These results confirmed the potentiality of LMR-based fiber biosensors for the detection of small molecules, such as PFOA, in synthetic samples.

T Kumar Dey, S Tombelli, P Biswas, A Giannetti, N Basumallick, F Baldini, S Bandyopadhyay, C Trono, Label-free immunosensing by long period fiber gratings at the lowest order, Optics & Laser Technology, 142, 107194, 2021

Abstract: A long period fiber grating has been fabricated by combining two sensitivity enhancement techniques, the turn around point (TAP) and the maximum enhancement of the evanescent field by obtaining the lowest order cladding mode (LP0,2 cladding mode). A label-free immunoglobulin G/anti immunoglobulin G (IgG/anti-IgG) immunoassay in serum has been performed with the sensor inside a thermostated closed flow cell. Real time kinetics of the IgG and anti-IgG binding has been monitored and evaluated. By measuring the dual peak resonance of the LP_0,2 cladding mode, a limit of detection of 0.16 ng/ml (1.06 pM) for anti-IgG has been achieved.

N Cennamo, C Trono, A Giannetti, F Baldini, A Minardo, L Zeni, S Tombelli, Biosensors exploiting unconventional platforms: the case of plasmonic light-diffusing fibers, Sens Actuat  B, 337, 129771, 2021

Abstract: In this work, an unconventional light-diffusing fiber (LDF) plasmonic platform is exploited to efficiently monitor a bio-interaction. The surface plasmon resonance (SPR)-LDF sensor is used to detect the immunoglobulin G (IgG)/anti-IgG interaction as exemplifying bioassay. The IgG bioreceptor was deposited on the gold surface of the SPR-LDF platform, and the biological target was transported through a customized thermo-stabilized flow cell by means of a buffer fluid. Moreover, to test the usability of the proposed SPR-LDF biosensor also for immunosensing in complex matrices, a second assay was conducted by immobilizing anti-C-reactive protein (CRP) and detecting different concentrations of CRP in human serum. An innovative data analysis approach for the extraction of the best value of the resonance wavelength was developed and applied. The obtained results reveal that the unconventional platform represented by the LDF can be successfully exploited for the implementation of SPR-based biosensors with very good performances, also when compared to more common SPR systems.

F Esposito, L Sansone, A Srivastava, F Baldini, S Campopiano, F Chiavaioli, M Giordano, A Giannetti, A Iadicicco, Long period grating in double cladding fiber coated with graphene oxide as high-performance optical platform for biosensing, Biosensors and Bioelectronics, 172, 112747, 2021

Abstract: In this work, the development and testing of a novel fiber-optic based label-free biosensor is presented, whose performance were verified through the detection of C-reactive protein (CRP) in serum. The device is based on a long period grating fabricated in a double cladding fiber with a W-shaped refractive index (RI) profile. As a result, the working point of the device was tuned to the mode transition region by chemical etching of the outer fiber cladding, obtaining a significant enhancement of the RI sensitivity and an excellent visibility of the grating resonances due to the mode transition in an all-silica structure. The fiber transducer was coated with a nanometric thin layer of graphene oxide in order to provide functional groups for the covalent immobilization of the biological recognition element. A very low limit of detection of about 0.15 ng/mL was obtained during the detection of CRP in serum, and a large working range (1 ng/mL – 100 μg/mL) of clinical relevance has been also achieved.

TK Dey, S Tombelli, P Biswas, A Giannetti, N Basumallick, F Baldini, S Bandyopdhayay, C Trono, Analysis of the Lowest Order Cladding Mode of Long Period Fiber Gratings near Turn Around Point, J Lightwave Technology, 39 (12), 4006-4012, 2021

Abstract: A long period fiber grating (LPFG) sensor has been fabricated, after theoretical analysis, obtaining the maximum enhancement of the evanescent field working with the lowest order cladding mode (LP_0,2 cladding mode) near turn around point. Both the wavelengths of the dual peak resonance and the coupling conditions of the mode have been analyzed in detail. A flow cell has been fabricated to characterize the sensor in term of surrounding refractive index (SRI) sensitivity. The sensitivity of the sensor resulted to be ∼8751 nm/SRIU, with a resolution in the order of 10^−5, within the SRI range of 1.333 to 1.3335.

C Trono, F Valeri, F Baldini, Discretized superimposed optical fiber long-period gratings, Optics Letters 45 (4), 807-810, 2020

Abstract: A novel technique, to the best of our knowledge, for the inscription of superimposed long-period gratings with arbitrary grating pitches is proposed and experimentally validated. The technique is based on the discretization of an ideal continuous sinusoidal refractive index (RI) pattern with a step function. The RI variation is induced by means of the irradiation of a photosensitive fiber with a 248 nm UV laser beam. The nonlinear relation between the induced RI change and the UV fluence was experimentally derived. Two superimposed long-period grating (LPGs) with different grating pitches have been realized with the discretization technique; the transmission spectrum was compared with that of two superimposed LPGs obtained with the traditional square wave RI modulation. The validity of the proposed technique was demonstrated by the better spectral characteristics of the discretized superimposed LPGs.

P Zubiate, A Urrutia, C R Zamarreño, J Egea-Urra, J Fernández-Irigoyen, A Giannetti, F Baldini, S Díaz, IR Matias, F J Arregui, E Santamaría, F Chiavaioli, I Del Villar, Fiber-based early diagnosis of venous thromboembolic disease by label-free D-dimer detection, Biosensors & Bioelectronics, 2, 100026, 2019

Abstract: D-dimer is a useful diagnostic biomarker for deep vein thrombosis or pulmonary embolism, collectively referred to as venous thromboembolism (VTE). The ability to detect in real-time the amount of D-dimer with a fast and reliable method is a key step to anticipate the appearance of these diseases. Here, the results of a highly specific and sensitive biosensor for the detection of D-dimer based on lossy mode resonance in fiber optics are presented. The unique features of specialty fibers in light management integrated with microfluidics allow detecting D-dimer in human serum with a detection limit of 100 ng/mL, a value 5-fold below the clinical cutoff value. Comparison of the results achieved with mass-spectrometry-based proteomics, which allows for the identification of beta- and gamma-chains of fibrinogen, demonstrates the ability of our platform to specifically (>90%) recognize D-dimer. Therefore, this technology potentially represents a paradigm shift in the development of a simple, high-specificity and label-free biosensing platform, which can be applied to speed up diagnostic healthcare processes of venous thromboembolism toward an early diagnostic and personalized treatment system.

I Del Villar, O Fuentes, F Chiavaioli, JM Corres, I R Matias, Optimized Strain Long- Period Fiber Grating (LPFG) Sensors Operating at the Dispersion Turning Point, Journal of Lightwave Technology, 36(11), 2240-2247, 2018.

Abstract: Two phenomena for enhancing the sensitivity of long-period fiber gratings are combined toward an increase of the sensitivity to strain of this type of devices: the dispersion turning point (DTP) and the cladding diameter reduction by an etching process. The results prove that sensitivities up to 20 pm/με can be attained, which is a ten-fold improvement compared to the previous works. The sensitivity in the grating region, which is subjected to etching, does not depend on the order of the cladding mode responsible for the attenuation bands generated in the transmission spectrum, but on the proximity to the DTP for each mode order. On the other hand, the sensitivity to strain of the global structure, including the region without etching, can be increased for lower order modes in a perceptible way if the length of the etched region is smaller compared to the fiber region under stress. The experimental results are supported with simulations based on coupled-mode theory and on FIMMWAVE, which allows understanding the phenomena involved during the sensing process.

G Frigenti, M Arjmand, A Barucci, F Baldini, S Berneschi, D Farnesi, M Gianfreda, S Pelli, S Soria, A Aray, Y Dumeige, P Féron, G Nunzi Conti, Coupling analysis of high Q resonators in add-drop configuration through cavity ringdown spectroscopy, Journal of Optics, 20 (6), 065706, 2018

Abstract: An original method able to fully characterize high-Q resonators in an add-drop configuration has been implemented. The method is based on the study of two cavity ringdown (CRD) signals, which are produced at the transmission and drop ports by wavelength sweeping a resonance in a time interval comparable with the photon cavity lifetime. All the resonator parameters can be assessed with a single set of simultaneous measurements. We first developed a model describing the two CRD output signals and a fitting program able to deduce the key parameters from the measured profiles. We successfully validated the model with an experiment based on a fiber ring resonator of known characteristics. Finally, we characterized a high-Q, home-made, MgF2 whispering gallery mode disk resonator in the add-drop configuration, assessing its intrinsic and coupling parameters.

F Chiavaioli, P Zubiate, I Del Villar, C R Zamarreño, A Giannetti, S Tombelli, C Trono, F J Arregui, IR Matias, F Baldini, Femtomolar Detection by Nanocoated Fiber Label-Free Biosensors, ACS Sensors, 3 (5), 936-943, 2018

Abstract: The advent of optical fiber-based biosensors combined with that of nanotechnologies has provided an opportunity for developing in situ, portable, lightweight, versatile, and high-performance optical sensing platforms. We report on the generation of lossy mode resonances by the deposition of nanometer-thick metal oxide films on optical fibers, which makes it possible to measure precisely and accurately the changes in optical properties of the fiber-surrounding medium with very high sensitivity compared to other technology platforms, such as long period gratings or surface plasmon resonances, the gold standard in label-free and real-time biomolecular interaction analysis. This property, combined with the application of specialty structures such as D-shaped fibers, permits enhancing the light–matter interaction. SEM and TEM imaging together with X-EDS tool have been utilized to characterize the two films used, i.e., indium tin oxide and tin dioxide. Moreover, the experimental transmission spectra obtained after the deposition of the nanocoatings have been numerically corroborated by means of wave propagation methods. With the use of a conventional wavelength interrogation system and ad hoc developed microfluidics, the shift of the lossy mode resonance can be reliably recorded in response to very low analyte concentrations. Repeated experiments confirm a big leap in performance thanks to the capability to detect femtomolar concentrations in human serum, improving the detection limit by 3 orders of magnitude when compared with other fiber-based configurations. The biosensor has been regenerated several times by injecting sodium dodecyl sulfate, which proves the capability of sensor to be reused.

A Barucci, I A Grimaldi, G Persichetti, S Berneschi, S Soria, B Tiribilli, R Bernini, F Baldini, G Nunzi Conti, Selective coupling of Whispering Gallery modes in film coated micro-resonators, Optics Express, 26 (9), 11737-11741, 2018

Abstract: Whispering Gallery Mode (WGM) micro-resonators like microspheres or microtoroids are typically used as high-Q cavity substrate on which a functional film coating is deposited. In order to exploit the coating properties a critical step is the efficient excitation of WGMs mainly contained inside the deposited layer. We developed a simple method able to assess whether or not these modes are selectively excited. The method is based on monitoring the thermal shift of the excited resonance, which uniquely depends on the thermo-optic coefficient and on the thermal expansion coefficient of the material in which the mode is embedded.

S Bandyopadhyay, P Biswas, F Chiavaioli, T Kumar Dey, N Basumallick, C Trono, A Giannetti, S Tombelli, F Baldini, S Bandyopadhyay, Long-period fiber grating: a specific design for biosensing applications, Applied Optics, 56 (35), 9846-9, 2017

Abstract: In this paper, a detailed investigation on the modeling of long-period fiber grating (LPFG) sensors is discussed with the aim of providing a more realistic solution for their use in biosensing. Add-layer sensitivity, i.e., sensitivity of the sensor to an additional layer adhered onto the fiber surface, is quantified and a clear and complete analysis about the influence of the average thickness of the deposited biological sensing layers, as well as the change in refractive index of these layers, on the resonant wavelength of the cladding modes of an LPFG is provided. Add-layer sensitivity of LPFG sensors close to mode transition (MT) and also at turn-around point (TAP) are taken into account. Adsorbed layer thicknesses, as estimated from measured wavelength shifts of the LPFG, are found to have a good match with the values obtained through other measurement techniques. 853, 2017.

F Chiavaioli, F Baldini, C Trono, Manufacturing and spectral features of different type of long period fiber gratings: turn-around point, internally tilted and pseudo-random, Fibers, 5 (3), 29, 2017

Abstract: The manufacturing and spectral features of different types of long period fiber gratings (LPFGs), ranging from phase-shifted, turn-around point, and internally tilted gratings, to pseudo-random gratings, are described and discussed in detail. LPFGs were manufactured on boron-germanium co-doped photosensitive optical fibers with the point-by-point technique using an excimer KrF laser operating at 248 nm. The developed experimental setup to manufacture high-quality LPFGs was designed to totally customize any type of gratings with the possibility of setting different parameters, such as the grating period (or pitch), the number of grating planes, the number of laser shots for each plane, etc. Some important spectral features of the LPFGs’ spectra were taken into account. This allows realizing homemade devices useful in several fiber-based applications, such as optical filtering, coupling systems, random lasers, physical and chemical sensing, and biosensing.

S.Berneschi, F.Baldini, A.Cosci, D.Farnesi, G.Nunzi Conti, S.Tombelli, C.Trono, S. Pelli, A.Giannetti, Fluorescence biosensing in selectively photo–activated microbubbleresonators, Sens Actuat B, 242, 1057-1064, 2017

Abstract:A procedure for a spatially selective immobilization of antibodies, via photochemical activation, on theinner surface of optical microbubble resonators (OMBRs) is reported. The method is based on a suitableUV mask process. As proof of concept of this technique, immunoassay in photo-activated OMBR wasimplemented and fluorescence investigation was conducted. Moreover, the possible application of thisprocedure towards the development of a biochemical multiplexed detection system based on OMBR ispresented. Finally, the high Q factor values measured after the immunoassay event (>105) make thesephoto–activated OMBRs suitable devices for future label-free biosensor implementation.

P Biswas, F Chiavaioli, S Jana, N Basumallick, C Trono, A Giannetti, S Tombelli, A Mallick, F Baldini, S Bandyopadhyay, Design, fabrication and characterisation of silica-titania thin film coated over coupled long period fibre gratings: Towards bio-sensing applications, Sensors and Actuators B: Chemical, 253, 418-427, 2017

Abstract: In this paper we report on the application of over coupled long period fibre gratings (OCLPFGs) for detecting small changes in the surrounding refractive index (SRI). In addition, the optimization of a high RI (HRI) overlay deposited over the fibre for the adhesion of biological species is described. In this study, we utilised a sol-gel-based silica-titania thin film as HRI material deposited on the grating portion using the simple and versatile dip-coating (DC) technique. The OCLPFG-based sensor has been manufactured in such a way that the resonant band retains good visibility (close to the maximum coupling condition) in the transmission spectrum even after high thermal sintering of the coating material. Three different batches of OCLPFGs were produced using different withdrawal speeds and sol viscosities. By carefully tuning both the overlay thickness during the DC process and the RI of the sol-gel material during its preparation, it was possible to bring the sensor into the so-called transition mode working region, and thus maximize the sensing performance in terms of SRI changes. All devices were characterized as optical refractometers in the RI range of interest for a bio-sensing application (from 1.33 to 1.34) and, after a suitable bio-functionalisation process, one of them was used to implement a classical receptor-analyte biosensor.

S Berneschi, F Baldini, A Cosci, D Farnesi, G Nunzi Conti, S Tombelli, C Trono, S  Pelli, A Giannetti, Fluorescence biosensing in selectively photo–activated microbubble resonators, Sensors and Actuators B: Chemical, 242, 1057-1064, 2017

Abstract: IA procedure for a spatially selective immobilization of antibodies, via photochemical activation, on the inner surface of optical microbubble resonators (OMBRs) is reported. The method is based on a suitable UV mask process. As proof of concept of this technique, immunoassay in photo-activated OMBR was implemented and fluorescence investigation was conducted. Moreover, the possible application of this procedure towards the development of a biochemical multiplexed detection system based on OMBR is presented. Finally, the high Q factor values measured after the immunoassay event (>105) make these photo–activated OMBRs suitable devices for future label-free biosensor implementation.

Aray A, Chiavaioli F, Arjmand M, Trono C, Tombelli S, Giannetti A, Cennamo N, Soltanolkotabi M, Zeni L, Baldini F (2016). “SPR-based plastic optical fibre biosensor for the detection of C-reactive protein in serum.” J. Biophotonics, vol. 9 (10), p. 1077-1084.

Abstract:A plastic optical fibre biosensor based on surface plasmon resonance for the detection of C-reactive protein (CRP) in serum is proposed. The biosensor was integrated into a home-made thermo-stabilized microfluidic system that allows avoiding any thermal and/or mechanical fluctuation and maintaining the best stable conditions during the measurements. A working range of 0.006–70 mg L–1 and a limit of detection of 0.009 mg L–1 were achieved. These results are among the best compared to other SPR-based biosensors for CRP detection, especially considering that they were achieved in a real and complex medium, i.e. serum. In addition, since the sensor performances satisfy those requested in physiologically-relevant clinical applications, the whole biosensing platform could well address high sensitive, easy to realize, real-time, label-free, portable and low cost diagnosis of CRP for future lab-on-a-chip applications.

N. Cennamo, F. Chiavaioli, C.Trono, S.Tombelli, A.Giannetti, F. Baldini, L. Zeni: A Complete Optical Sensor System Based on a POF-SPR Platform and a Thermo-Stabilized Flow Cell for Biochemical Applications, Sensors, 16 (2), 196, 2016

Abstract:An optical sensor platform based on surface plasmon resonance (SPR) in a plastic optical fiber (POF) integrated into a thermo-stabilized flow cell for biochemical sensing applications is proposed. This device has been realized and experimentally tested by using a classic receptor-analyte assay. For this purpose, the gold surface of the POF was chemically modified through the formation of a self-assembling monolayer. The surface robustness of the POF-SPR platform has been tested for the first time thanks to the flow cell. The experimental results show that the proposed device can be successfully used for label-free biochemical sensing. The final goal of this work is to achieve a complete, small-size, simple to use and low cost optical sensor system. The whole system with the flow cell and the optical sensor are extensively described, together with the experimental results obtained with an immunoglobulin G (IgG)/anti-IgG assay.

M.Arjmand, F.Chiavaioli,S.Berneschi,F.Baldini,M.Soltanolkotabi, C.Trono: Effect of induced inner curvature on refractive index sensitivity in internally tilted long-period gratings, Optics Letters, 41 (7), 1443-1446, 2016

Abstract:A new complete analysis of the effect of induced inner curvature on refractive index (RI) sensitivity in internally tilted long-period gratings (ITLPGs) is presented. The responses in terms of RI sensitivity of a standard LPG and different ITLPGs with curvature values between 15 and 19  m-1 were compared. The analysis suggests first, that the larger the induced curvature, the greater the RI sensitivity; and second, that the RI sensitivity exponentially increases with both the curvature and cladding mode order. RI sensitivity greater than 100 nm RIU-1 can be attained with curvature greater than 25   m-1 for LP06 mode. Conversely, the temperature sensitivity of ITLPGs is comparable to standard LPGs for the considered cladding mode order.

A Cosci, S Berneschi, A Giannetti, D Farnesi, F Cosi, F Baldini, G Nunzi Conti, S Soria, A Barucci, G Righini, S Pelli, Resonance frequency of optical microbubble resonators: Direct measurements and mitigation of fluctuations, Sensors, 16(9), 1405, 2016

Abstract:  This work shows the improvements in the sensing capabilities and precision of an Optical Microbubble Resonator due to the introduction of an encaging poly(methyl methacrylate) (PMMA) box. A frequency fluctuation parameter σ was defined as a score of resonance stability and was evaluated in the presence and absence of the encaging system and in the case of air- or water-filling of the cavity. Furthermore, the noise interference introduced by the peristaltic and the syringe pumping system was studied. The measurements showed a reduction of σ in the presence of the encaging PMMA box and when the syringe pump was used as flowing system.

F. Chiavaioli, P. Biswas, C. Trono, S. Jana, S. Bandyopadhyay, N. Basumallick, A. Giannetti, S. Tombelli, S. Bera, A. Mallick, F. Baldini: Sol-gel based titania-silica thin film overlay for long period fiber grating-based biosensors, Analytical Chemistry, 87 (24) 12024-120131, 2015

Abstract:An evanescent wave optical fiber biosensor based on titania–silica-coated long period grating (LPG) is presented. The chemical overlay, which increases the refractive index (RI) sensitivity of the sensor, consists of a sol–gel-based titania–silica thin film, deposited along the sensing portion of the fiber by means of the dip-coating technique. Changing both the sol viscosity and the withdrawal speed during the dip-coating made it possible to adjust the thickness of the film overlay, which is a crucial parameter for the sensor performance. After the functionalization of the fiber surface using a methacrylic acid/methacrylate copolymer, an antibody/antigen (IgG/anti-IgG) assay was carried out to assess the performance of sol–gel based titania–silica-coated LPGs as biosensors. The analyte concentration was determined from the wavelength shift at the end of the binding process and from the initial binding rate. This is the first time that a sol–gel based titania–silica-coated LPG is proposed as an effective and feasible label-free biosensor. The specificity of the sensor was validated by performing the same model assay after spiking anti-IgG into human serum. With this structured LPG, detection limits of the order of tens of micrograms per liter (10–11 M) are attained.

D.Farnesi, F.Chiavaioli, F.Baldini, G.C.Righini, S.Soria, C.Trono, G.Nunzi Conti: Quasi-distributed and wavelength selective addressing of optical micro-resonators based on long period fiber gratings, Optics Express, 23 (16), 21175- 21180, 2015

Abstract:A novel all-in-fiber method for coupling light to high-Q silica whispering gallery mode (WGM) optical micro-resonators is presented, which is based on a pair of long period fiber gratings (LPGs) written in the same silica fiber, along with a thick fiber taper (15–18 μm in waist) in between the LPGs. The proposed coupling structure is robust and can be replicated many times along the same fiber simply cascading LPGs with different bands. Typical Q-factors of the order of 108 and total coupling efficiency up to 60% were measured collecting the resonances of microspheres or microbubbles at the fiber end. This approach uniquely allows quasi-distributed and wavelength selective addressing of different micro-resonators along the same fiber.

C.Berrettoni, C.Trono, V.Vignoli, F.Baldini: Fibre Tip Sensor with Embedded FBG-LPG for Temperature and Refractive Index Determination by means of the Simple Measurement of the FBG Characteristics, Journal of Sensors, 491391 (8 pages), 2015

Abstract:A novel optical fibre sensing system based on a hybrid long period grating (LPG) and Bragg grating (FBG) configuration is proposed and demonstrated experimentally. The hybrid configuration, which uses the difference in temperature and refractive index (RI) different response of a Bragg grating and a long period grating, makes it possible to discriminate simultaneously the temperature and the refractive index of different aqueous solutions. RI (1.33 RIU–1.40 RIU) and temperature (21°C–28°C) working ranges have been experimentally determined. Experimental results show that the maximum accuracy in the refractive index measurement (0.004 RIU) with temperature compensation has been achieved within the working ranges.

F.Chiavaioli, P.Biswasb, C.Trono, S.Bandyopadhyay, A.Giannetti, S.Tombelli, N.Basumallick, K.Dasgupta, and F.Baldini: Towards sensitive label-free immunosensing by means of turn-around point long period fiber gratings, Biosens. Bioelectron. 60, pp 305-310, 2014

Abstract:Long period fiber gratings have been effectively used in the field of biochemical sensing since a few years. Compared to other well-known label-free optical approaches, long period gratings (LPGs) take advantage of the typical peculiarity of optical fibers. Coupling the propagating core mode with a high-order cladding mode near its turn-around point (TAP) was the strategy adopted to achieve good performances without additional coatings, except for the sensing and selective biolayer deposited on the fiber. Both the modeling and manufacturing of TAP LPGs were discussed. After the functionalization of the fiber surface with the deposition of a Eudragit L100 copolymer layer followed by immunoglobulin G (IgG) covalent immobilization, an IgG/anti-IgG bioassay was implemented along the grating region and the kinetics of antibody/antigen interaction was analyzed. A quantitative comparison between a TAP LPG and a non-TAP LPG was carried out to highlight the improvement of the proposed immunosensor. The real effectiveness and feasibility of an LPG-based biosensor were demonstrated by using a complex matrix consisting of human serum, which also confirmed the specificity of the assay, and a limit of detection of 70 μg L(-1) (460 pM) was achieved.

I. A. Grimaldi, S. Berneschi, G. Testa, F. Baldini, G. Nunzi Conti, and R. Bernini: Polymer based planar coupling of self-assembled bottle microresonators, Appl. Phys. Lett., 105, 231114 1-4, 2014

Abstract:The investigation of a simple and self-assembling method for realizing polymeric micro-bottle resonators is reported. By dispensing precise amounts of SU-8 onto a cleaved optical fiber, employed as mechanical support, bottle microcavities with different shapes and diameters are fabricated. The balancing of surface energy between glass fiber and polymeric microresonator with surface tension of SU-8 confers different shape to these microstructures. Planar single-mode SU-8 based waveguide, realized on polymethylmethacrylate, is chosen for exciting the micro-bottle resonators by evanescent wave. The reliability of the fabrication process and the shape of the bottle microcavities are investigated through optical analysis. We observe whispering gallery modes in these resonant microstructures by a robust coupling with single mode planar waveguides around 1.5 μm wavelength. The resonance spectra of micro-bottle resonators and the spectral characteristics, such as Quality-factor (Q factor) and free spectral range, are evaluated for all the realized microstructures. SU-8 micro-bottle resonators show high Q-factors up to 3.8 × 104 and present a good mechanical stability. These features make these microcavities attractive for sensing and/or lasing applications in a planar platform.

F. Chiavaioli, C. Trono, A. Giannetti, M. Brenci, F. Baldini: Characterisation of a label-free biosensor based on long period grating, J. Biophotonics . 7 (5), 312–322 (2014)

Abstract:Optical fibre gratings, especially long period gratings, have been recently proposed as optical devices for biochemical sensing. A biochemical interaction along the grating portion induces a refractive index change and hence a change in the fiber transmission spectrum. This provides an alternative methodology with respect to other label-free optical approaches, such as surface plasmon resonance, interferometric configurations and optical resonators. The fibre biofunctionalization has been carried out by means of a novel chemistry using Eudragit L100 copolymer as opposed to the commonly used silanization procedure. Antigen-antibody interaction has been analysed by means of an IgG/anti-IgG bioassay. The biosensor was fully characterised, monitoring the kinetics during the antibody immobilization and the antigen interaction and achieving the calibration curve of the assay. A comparison of the biosensor performance was made by using two different long period gratings with distinct periods.

F, Chiavaioli, C, Trono, F, Baldini: Specially designed long period grating with internal geometric bending for enhanced refractive index sensitivity, Appl Phys Lett, 102 (23), pp. 231109-1 - 231109-4, 2013

Abstract:We propose a long period grating (LPG) characterized by specially designed refractive index (RI) profile in which each grating plane is tilted at increasing angles, as moving away from the center of symmetry of the structure towards its both edges. This internally manufactured geometric structure, which basically simulates the bending of an optical fiber, increases the RI sensitivity of an LPG to the external medium. We experimentally demonstrate a three-fold improvement in the RI sensitivity, thus providing the basis for another step forward in the field of RI sensors based on optical fiber gratings.

A. Giannetti, S. Berneschi, F. Baldini, F. Cosi, G. Nunzi Conti, S. Soria: Performance of Eudragit Coated Whispering Gallery Mode Resonator-Based Immunosensors, Sensors, 12, pp. 14604-14611, 2012

Abstract:Whispering gallery mode resonators (WGMR) are an efficient tool for the realization of optical biosensors. A high Q factor preservation is a crucial requirement for good biosensor performances. In this work we present an Eudragit®L100 coated microspherical WGMR as an efficient immunosensor. The developed resonator was morphologically characterized using fluorescence microscopy. The functionalization process was tuned to preserve the high Q factor of the resonator. The protein binding assay was optically characterized in terms of specificity in buffer solution.

P. Pilla, C. Trono, F. Baldini, F. Chiavaioli, M. Giordano, A. Cutolo, A. Cusano: Giant sensitivity of Long Period Gratings in transition mode near the dispersion turning point: an integrated design approach, Opt.Lett., 37 (19), pp. 4152-4155, 2012

Abstract:We report an original design approach based on the modal dispersion curves for the development of long period gratings in transition mode near the dispersion turning point exhibiting ultrahigh refractive index sensitivity. The theoretical model predicting a giant sensitivity of 9900 nm per refractive index unit in a watery environment was experimentally validated with a result of approximately 9100 nm per refractive index unit around an ambient index of 1.3469. This result places thin film coated LPGs as an alternative to other fiber-based technologies for high-performance chemical and biological sensing applications.

S. Surdo, S. Merlo, F. Carpignano, L. M. Strambini, C. Trono, A. Giannetti, F. Baldini, G. Barillaro: Optofluidic microsystems with integrated vertical one-dimensional photonic crystals for chemical analysis, Lab Chip, 12, pp. 4403–4415, 2012

Abstract:In this work, we report all-silicon, integrated optofluidic microsystems (OFMs) fabricated by electrochemical micromachining (ECM) technology, in which high aspect-ratio (HAR) photonic crystal (PhC) devices (i.e. micromirrors, optical cavities) are integrated by one-etching-step, together with microfluidic reservoirs/channels, for the infiltration of liquids in the PhC air gaps, and with fiber grooves for alignment/positioning of readout optical fibers in front of the PhC, on the same silicon die. This has not previously been reported in the literature, and opens up new ground in, though not limited to, the optofluidics field, due to the low-cost and high-flexibility of the ECM technology that allows optofluidic microsystem fabrication to be performed in any lab. Optofluidic characterization of PhC-OFMs by both capillary-action and pressure-driven operations is carried out through the measurement of the reflectivity spectra of HAR-PhCs upon injection of liquids featuring different refractive index values in the HAR-PhC air gaps, by using readout optical fibers positioned in the on-chip fiber grooves. High sensitivity and good limit of detection of PhC-OFMs are obtained for both capillary-action and pressure-driven operations. A best sensitivity value of 670 nm/RIU and a worst-case limit of detection of the order of 10-3 RIU are measured, the former being comparable to state-of-the-art integrated refractive index sensors and the latter being limited by constraints of the experimental setup. The proof of concept about the biosensing potential of PhC-OFMs is given by successfully carrying out a sandwich assay based on antigen–antibody interactions for the detection of the C-reactive protein (CRP) at a concentration value of 10 mg L-1, which represents the boundary level between physiological and pathological conditions.

S. Soria, F. Baldini, S. Berneschi, F. Cosi, A. Giannetti, G. Nunzi Conti, S. Pelli, G.C. Righini, B. Tiribilli: High-Q polymer-coated microspheres for immunosensing applications, Optics Express, 17, pp. 14694–14699, 2009

Abstract: Homogeneous polymeric thin layers have been used as functionalizing agents on silica microspherical resonators in view of the implementation of an immunosensor. We have characterized the microspheres functionalised with poly-L-lactic acid and Eudragit® L100, as an alternative to the commonly used 3-Aminopropyltrimethoxysilane. It is shown that polymeric functionalization does not affect the high quality factor (Q greater than 107) of the silica microspheres, and that the Q factor is about 3x105 after chemical activation and covalent binding of immunogammaglobulin (IgG). This functionalizing process of the microresonator constitutes a promising step towards the achievement of an ultra sensitive immunosensor.

Farnesi D, Chiavaioli F, Righini GC, Soria S, Trono C, Jorge P, Nunzi Conti G (2014). “Long period grating-based fiber coupler to whispering gallery mode resonators”. Optics Letters, vol. 39 (22), p. 6525-6528.

Abstract:We present a new method for coupling light to high-Q silica whispering gallery mode resonators (WGMs) that is based on long period fiber gratings (LPGs) written in silica fibers. An LPG allows selective excitation of high-order azimuthally symmetric cladding modes in a fiber. Coupling of these cladding modes to WGMs in silica resonators is possible when partial tapering of the fiber is also implemented in order to reduce the optical field size and increase its external evanescent portion. Importantly, the taper size is about one order of magnitude larger than that of a standard fiber taper coupler. The suggested approach is therefore much more robust and useful especially for practical applications. We demonstrate coupling to high-Q silica microspheres and microbubbles detecting the transmission dip at the fiber output when crossing a resonance. An additional feature of this approach is that by cascading LPGs with different periods, a wavelength selective addressing of different resonators along the same fiber is also possible.

F. Baldini, M. Brenci, F. Chiavaioli, A. Giannetti, C Trono: Optical fibre gratings as tools for chemical and biochemical sensing,Anal. Bioanal. Chem., 402, pp. 109-116, 2012

Abstract:Optical fibre gratings have recently been suggested as optical platforms for chemical and biochemical sensing. On the basis of the measurement of refractive index changes induced by a chemical and biochemical interaction in the transmission spectrum along the fibres, they are proposed as a possible alternative to the other label-free optical approaches, such as surface plasmon resonance and optical resonators. The combination of the use of optical fibres with the fact that the signal modulation is spectrally encoded offers multiplexing and remote measurement capabilities which the other technology platforms are not able to or can hardly offer. The fundamentals of the different types of optical fibre gratings are described and the performances of the chemical and biochemical sensors based on this approach are reviewed. Advantages and limitations of optical fibre gratings are considered, with a look at new perspectives for their utilization in the field.

C. Trono, F. Baldini, M. Brenci, F. Chiavaioli and M. Mugnaini, Flow cell for strain- and temperature-compensated refractive index measurements by means of cascaded optical fibre long period and Bragg gratings, Meas Sci Technol, 22, pp. 075204, 2011

Abstract:An optical fibre sensing system based on a hybrid cascaded long period grating (LPG) and fibre Bragg grating configuration and a thermo-stabilized flow cell for refractometric measurements is proposed. The system makes it possible to measure, and thus to cancel, the LPG cross-sensitivities to strain, temperature and fibre bending. The experimental results show that the proposed system provides satisfactory performances as far as the refractive index sensitivity and resolution are concerned. The maximum sensor sensitivity and resolution are 3120 nm/RIU and 2 × 10−5 RIU, respectively. The whole system with its flow cell and the gratings fabrication are extensively described, together with the acquisition and data processing. The stability of the sensor for several hours was also tested. We believe that the proposed system can be successfully used for label-free chemical/biochemical sensing.