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

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

Date: 2012-2014.

Reference number: 22/EU/Italy/CNR/proj./2012.

Activity period: from 2013 up to 2015.

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

SENS4BIO 

Main Publications on the subject

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, 2016, in press

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.

Grimaldi I. A., Berneschi S., Testa G., Baldini F., Nunzi Conti G., Bernini R., Whispering gallery mode in self-assembled bottle microresonators coupled to planar waveguide, Proc.SPIE 9750, Integrated Optics: Devices, Materials, and Technologies XX (13-18 Febbraio 2016, San Francisco, USA), pp. 97501H-1 -97501H-1, 2016

Abstract:Polymer micro-bottle resonators are fabricated by means of a simple self-assembling process. High optical quality SU-8 negative resist is chosen as bottle material. SU-8 is dispensed onto a fiber stem and is kept in rotation during the UV photo-polymerization. Micro-bottles with different geometrical sizes have been obtained by changing the SU-8 dispensed volume, with a repeatability of about 2%. Planar waveguides is chosen as interrogation system for our SU-8 self-assembled micro-bottle resonators. The highest quality factor Q associated to the resonance peak in the transmission spectrum is about 3.8 × 104.

Berneschi S., Baldini F., Barucci A., Cosci A., Cosi F., Farnesi D., Nunzi Conti G., Righini G. C., Soria S., Tombelli S., Trono C., Pelli S., Giannetti A. , Localized biomolecules immobilization in optical microbubble resonators, Proc.SPIE 9727, Laser Resonators, Microresonators, and Beam Control XVIII (13-18 Febbraio 2016, San Francisco, USA), pp. 972719-1 -972719-8, 2016

Abstract:In order to optimize the performance of an optical microbubble resonator (OMBR) as biosensor, the chemical functionalization of its inner surface plays a key role. Here we report on a spatially selective photo – chemical procedure able to bind fluorescent biomolecules only in correspondence of the OMBR inner surface. This abruptly reduces the occurrence of an undesired specific biochemical bond event all along the microfluidic section of the device. The evidence of this method, which maintains high Q factor (> 105) for the OMBR in buffer solution, is proved by fluorescence microscopy and real time measurement of the resonance broadening. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

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.Giannetti, A. Barucci,S. Berneschi, A. Cosci, F. Cosi, D. Farnesi, G. Nunzi Conti, S. Pelli, S. Soria, S. Tombelli, C. Trono, G.C. Righini, F. Baldini, Optical micro-bubble resonators as promising biosensors, Proc.SPIE 9506, Optical Sensors (13-16 Aprile 2015, Prague, Czech Republic), pp. 950617-1 - 950617-7, 2015

Abstract:Recently, optical micro-bubble resonators (OMBRs) have gained an increasing interest in many fields of photonics thanks to their particular properties. These hollow microstructures can be suitable for the realization of label – free optical biosensors by combining the whispering gallery mode (WGM) resonator properties with the intrinsic capability of integrated microfluidics. In fact, the WGMs are morphology-dependent modes: any change on the OMBR inner surface (due to chemical and/or biochemical binding) causes a shift of the resonance position and reduces the Q factor value of the cavity. By measuring this shift, it is possible to obtain information on the concentration of the analyte to be detected. A crucial step for the development of an OMBR-based biosensor is constituted by the functionalization of its inner surface. In this work we report on the development of a physical and chemical process able to guarantee a good homogeneity of the deposed bio-layer and, contemporary, to preserve a high quality factor Q of the cavity. The OMBR capability of working as bioassay was proved by different optical techniques, such as the real time measurement of the resonance broadening after each functionalization step and fluorescence microscopy.

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.