Label-free biological and chemical sensors

Antibody oriented immobilization on gold using the reaction between carbon disulfide and amine groups and its application in immunosensing

Carbon disulfide (CS(2)) can spontaneously react with amine groups to form dithiocarbamates on gold surface, providing the possibility to immobilize some compounds with primary or secondary amine groups in one step. Using this principle, an immunosensor interface prepared for immunoglobulin G (IgG) sensing surface toward anti-IgG has been fabricated for the first time by simply immersing gold slides into a mixed aqueous solution of CS(2) and protein A, followed by incubation in immunoglobulin G solution. The reaction between CS(2) and protein A has been followed by UV-vis spectroscopy, whereas cyclic voltammetry has been employed in the characterization of the modified gold surface with CS(2) and protein A, both methods indicating that protein A immobilization is implemented by CS(2). Conventional ellipsometry, atomic force microscopy (AFM), as well as surface plasmon resonance (SPR) have been used to evaluate the specific binding of protein A with IgG and IgG with anti-IgG, revealing that IgG is specifically captured to form the biosensing interface, maintaining its bioactivity. Compared to direct adsorption of IgG on the gold surface, the IgG sensing surface constructed of CS(2) and protein A is far more sensitive to capture anti-IgG as its target molecule. In addition, the modified surface is proven to have good capability to inhibit nonspecific adsorption, as supported by control experiments using lysozyme and BSA. To conclude, antibody immobilization using this one-step method has potential as a simple and convenient surface modification approach for immunosensor development.

Recent advances in integrated photonic sensors

Nowadays, optical devices and circuits are becoming fundamental components in several application fields such as medicine, biotechnology, automotive, aerospace, food quality control, chemistry, to name a few. In this context, we propose a complete review on integrated photonic sensors, with specific attention to materials, technologies, architectures and optical sensing principles. To this aim, sensing principles commonly used in optical detection are presented, focusing on sensor performance features such as sensitivity, selectivity and rangeability. Since photonic sensors provide substantial benefits regarding compatibility with CMOS technology and integration on chips characterized by micrometric footprints, design and optimization strategies of photonic devices are widely discussed for sensing applications. In addition, several numerical methods employed in photonic circuits and devices, simulations and design are presented, focusing on their advantages and drawbacks. Finally, recent developments in the field of photonic sensing are reviewed, considering advanced photonic sensor architectures based on linear and non-linear optical effects and to be employed in chemical/biochemical sensing, angular velocity and electric field detection.

Configuration change of liquid crystal microdroplets coated with a novel polyacrylic acid block liquid crystalline polymer by protein adsorption

4-cyano-4'-pentylbiphenyl (5CB) microdroplets coated with PAA-b-LCP were successfully made via microfluidics. Their functionalized interfaces were examined for lysozyme and bovine serum albumin detection, which could be easily visualized with an optical microscope under crossed polarizers by the radial-to-bipolar configurational change of the 5CB. This transition was strongly dependent on the electrostatic states of the proteins and PAA chains, which were controlled by the pH of the water. This study demonstrated a simple approach for the development of a liquid crystal-based sensor for label-free detection of proteins, which may have diverse applications in various biomedical fields.

Tailoring the protein adsorption properties of whispering gallery mode optical biosensors

Label-free biosensor technologies have the potential to revolutionize environmental monitoring, medical diagnostics, and food safety evaluation processes due to their unique combinations of high-sensitivity signal transducers and high-specificity recognition elements. This enables their ability to perform real-time detection of deleterious compounds at extremely low concentrations. However, to further improve the biosensors' performance in complex environments, such as wastewater, blood, and urine, it is necessary to minimize nonspecific binding, which in turn will increase their specificity, and decrease the rate of false positives. In the present work, we illustrate the potential of combining emerging high-sensitivity optical signal transducers, such as whispering gallery mode (WGM) microcavities, with covalently bound poly(ethylene glycol) (PEG) coatings of varying thickness, as an effective treatment for the prevention of nonspecific protein adsorption onto the biosensor surface. We monitor the sensitivity of the coated biosensor, and investigate the effect of PEG chain length on minimizing nonspecific adsorption via protein adsorption studies. Experimental results confirm not only that PEG-functionalization reduces nonspecific protein adsorption to the surface of the sensor by as much as a factor of 4 compared to an initialized control surface, but also that chain length significantly impacts the nonfouling character of the microcavity surface. Surprisingly, it is the short chain PEG surfaces that experience the best improvement in specificity, unlike many other systems where longer PEG chains are preferred. The combination of WGM microcavities with PEG coatings tuned specifically to the device will significantly improve the overall performance of biosensor platforms, and enable their wider application in complex, real-world monitoring scenarios.

Mass transport effects in suspended waveguide biosensors integrated in microfluidic channels

Label-free optical biosensors based on integrated photonic devices have demonstrated sensitive and selective detection of biological analytes. Integrating these sensor platforms into microfluidic devices reduces the required sample volume and enables rapid delivery of sample to the sensor surface, thereby improving response times. Conventionally, these devices are embedded in or adjacent to the substrate; therefore, the effective sensing area lies within the slow-flow region at the floor of the channel, reducing the efficiency of sample delivery. Recently, a suspended waveguide sensor was developed in which the device is elevated off of the substrate and the sensing region does not rest on the substrate. This geometry places the sensing region in the middle of the parabolic velocity profile, reduces the distance that a particle must travel by diffusion to be detected, and allows binding to both surfaces of the sensor. We use a finite element model to simulate advection, diffusion, and specific binding of interleukin 6, a signaling protein, to this waveguide-based biosensor at a range of elevations within a microfluidic channel. We compare the transient performance of these suspended waveguide sensors with that of traditional planar devices, studying both the detection threshold response time and the time to reach equilibrium. We also develop a theoretical framework for predicting the behavior of these suspended sensors. These simulation and theoretical results provide a roadmap for improving sensor performance and minimizing the amount of sample required to make measurements.

Single beam grating coupled interferometry: high resolution miniaturized label-free sensor for plate based parallel screening

Grating Coupled Interferometry (GCI) using high quality waveguides with two incoupling and one outcoupling grating areas is introduced to increase and precisely control the sensing length of the device; and to make the sensor design suitable for plate-based multiplexing. In contrast to other interferometric arrangements, the sensor chips are interrogated with a single expanded laser beam illuminating both incoupling gratings simultaneously. In order to obtain the interference signal, only half of the beam is phase modulated using a laterally divided two-cell liquid crystal modulator. The developed highly symmetrical arrangement of the interferometric arms increases the stability and at the same time offers straightforward integration of parallel sensing channels. The device characteristics are demonstrated for both TE and TM polarized modes.

Optofluidic devices and applications in photonics, sensing and imaging

Optofluidics integrates the fields of photonics and microfluidics, providing new freedom to both fields and permitting the realization of optical and fluidic property manipulations at the chip scale. Optofluidics was formed only after many breakthroughs in microfluidics, as understanding of fluid behaviour at the micron level enabled researchers to combine the advantages of optics and fluids. This review describes the progress of optofluidics from a photonics perspective, highlighting various optofluidic aspects ranging from the device's property manipulation to an interactive integration between optics and fluids. First, we describe photonic elements based on the functionalities that enable fluid manipulation. We then discuss the applications of optofluidic biodetection with an emphasis on nanosensing. Next, we discuss the progress of optofluidic lenses with an emphasis on its various architectures, and finally we conceptualize on where the field may lead.

Cascaded Raman microlaser in air and buffer

Optical microcavities provide an intriguing platform for the development of low threshold microlasers based on nonlinear effects. Long photon lifetimes within the cavity translate to high circulating optical intensities, thereby reducing the lasing threshold. It is therefore possible to create lasers that can operate in complex environments. In the present work, we use a silica microsphere to demonstrate a cascaded Raman microlaser that operates in air and buffer with the first emission peak around 800 nm in both environments. As expected, the threshold in air is significantly lower than in buffer.

Serpentine low loss trapezoidal silica waveguides on silicon

We report the fabrication and characterization of straight and serpentine low loss trapezoidal silica waveguides integrated on a silicon substrate. The waveguide channel was defined using a dual photo-lithography and buffered HF etching and isolated from the silicon substrate using an isotropic silicon etchant. The waveguide is air-clad and thus has a core-cladding effective index contrast of approximately 25%. Measured at 658, 980 and 1550 nm, the propagation loss was found to be 0.69, 0.59, and 0.41 dB/cm respectively, with a critical bending radius less than 375 μm. The waveguide's polarization behavior was investigated both theoretically and experimentally. Additionally, the output power shows a linear response with input power up to 200 mW.

Fabrication of nanofluidic biochips with nanochannels for applications in DNA analysis

With the development of nanotechnology, great progress has been made in the fabrication of nanochannels. Nanofluidic biochips based on nanochannel structures allow biomolecule transport, bioseparation, and biodetection. The domain applications of nanofluidic biochips with nanochannels are DNA stretching and separation. In this Review, the general fabrication methods for nanochannel structures and their applications in DNA analysis are discussed. These representative fabrication approaches include conventional photolithography, interference lithography, electron-beam lithography, nanoimprint lithography and polymer nanochannels. Other nanofabrication methods used to fabricate unique nanochannels, including sub-10-nm nanochannels, single nanochannels, and vertical nanochannels, are also mentioned. These nanofabrication methods provide an effective way to form nanoscale channel structures for nanofluidics and biosensor devices for DNA separation, detection, and sensing. The broad applications of nanochannels and future perspectives are also discussed.

Chemical silicon surface modification and bioreceptor attachment to develop competitive integrated photonic biosensors

Methodology for the functionalization of silicon-based materials employed for the development of photonic label-free nanobiosensors is reported. The studied functionalization based on organosilane chemistry allowed the direct attachment of biomolecules in a single step, maintaining their bioavailability. Using this immobilization approach in probe microarrays, successful specific detection of bacterial DNA is achieved, reaching hybridization sensitivities of 10 pM. The utility of the immobilization approach for the functionalization of label-free nanobiosensors based on photonic crystals and ring resonators was demonstrated using bovine serum albumin (BSA)/anti-BSA as a model system.

Opportunities for improved serodiagnosis of human tuberculosis, bovine tuberculosis, and paratuberculosis

Mycobacterial infections—tuberculosis (TB), bovine tuberculosis (bTB), and Johne's disease (JD)—are major infectious diseases of both human and animals. Methods presently in use for diagnosis of mycobacterial infections include bacterial culture, nucleic acid amplification, tuberculin skin test, interferon-γ assay, and serology. Serological tests have several advantages over other methods, including short turn-around time, relatively simple procedures, and low cost. However, current serodiagnostic methods for TB, bTB and JD exhibit low sensitivity and/or specificity. Recent studies that have aimed to develop improved serodiagnostic tests have mostly focused on identifying useful species-specific protein antigens. A review of recent attempts to improve diagnostic test performance indicates that the use of multiple antigens can improve the accuracy of serodiagnosis of these mycobacterial diseases. Mycobacteria also produce a variety of species-specific nonprotein molecules; however, only a few such molecules (e.g., cord factor and lipoarabinomannan) have so far been evaluated for their effectiveness as diagnostic antigens. For TB and bTB, there has been recent progress in developing laboratory-free diagnostic methods. New technologies such as microfluidics and “Lab-on-Chip” are examples of promising new technologies that can underpin development of laboratory-free diagnostic devices for these mycobacterial infections.

Leveraging bimodal kinetics to improve detection specificity

Optical microcavities are high sensitivity transducers able to detect single nanoparticles and molecules. However, the specificity of detection is dependent on the availability of an appropriate targeting moiety with minimal cross-reactivity. In the present work, an alternative approach is shown. Namely, using biotin-functionalized toroidal microcavities, the dissociation constant of biotin to two different streptavidin complexes (free and polystyrene bead) is determined. Based on the difference in affinity and in mass transport, the two complexes are identified from a mixture. By leveraging information in the binding site, improved specificity can be achieved.

Attaching biological probes to silica optical biosensors using silane coupling agents

In order to interface with biological environments, biosensor platforms, such as the popular Biacore system (based on the Surface Plasmon Resonance (SPR) technique), make use of various surface modification techniques, that can, for example, prevent surface fouling, tune the hydrophobicity/hydrophilicity of the surface, adapt to a variety of electronic environments, and most frequently, induce specificity towards a target of interest. These techniques extend the functionality of otherwise highly sensitive biosensors to real-world applications in complex environments, such as blood, urine, and wastewater analysis. While commercial biosensing platforms, such as Biacore, have well-understood, standard techniques for performing such surface modifications, these techniques have not been translated in a standardized fashion to other label-free biosensing platforms, such as Whispering Gallery Mode (WGM) optical resonators. WGM optical resonators represent a promising technology for performing label-free detection of a wide variety of species at ultra-low concentrations. The high sensitivity of these platforms is a result of their unique geometric optics: WGM optical resonators confine circulating light at specific, integral resonance frequencies. Like the SPR platforms, the optical field is not totally confined to the sensor device, but evanesces; this "evanescent tail" can then interact with species in the surrounding environment. This interaction causes the effective refractive index of the optical field to change, resulting in a slight, but detectable, shift in the resonance frequency of the device. Because the optical field circulates, it can interact many times with the environment, resulting in an inherent amplification of the signal, and very high sensitivities to minor changes in the environment. To perform targeted detection in complex environments, these platforms must be paired with a probe molecule (usually one half of a binding pair, e.g. antibodies/antigens) through surface modification. Although WGM optical resonators can be fabricated in several geometries from a variety of material systems, the silica microsphere is the most common. These microspheres are generally fabricated on the end of an optical fiber, which provides a "stem" by which the microspheres can be handled during functionalization and detection experiments. Silica surface chemistries may be applied to attach probe molecules to their surfaces; however, traditional techniques generated for planar substrates are often not adequate for these three-dimensional structures, as any changes to the surface of the microspheres (dust, contamination, surface defects, and uneven coatings) can have severe, negative consequences on their detection capabilities. Here, we demonstrate a facile approach for the surface functionalization of silica microsphere WGM optical resonators using silane coupling agents to bridge the inorganic surface and the biological environment, by attaching biotin to the silica surface. Although we use silica microsphere WGM resonators as the sensor system in this report, the protocols are general and can be used to functionalize the surface of any silica device with biotin.

Determination of surface concentrations of individual molecule-layers used in nanoscale biosensors by in situ ATR-FTIR spectroscopy

For the development of nanowire sensors for chemical and medical detection purposes, the optimal functionalization of the surface is a mandatory component. Quantitative ATR-FTIR spectroscopy was used in situ to investigate the step-by-step layer formation of typical functionalization protocols and to determine the respective molecule surface concentrations. BSA, anti-TNF-α and anti-PSA antibodies were bound via 3-(trimethoxy)butylsilyl aldehyde linkers to silicon-oxide surfaces in order to investigate surface functionalization of nanowires. Maximum determined surface concentrations were 7.17 × 10(-13) mol cm(-2) for BSA, 1.7 × 10(-13) mol cm(-2) for anti-TNF-α antibody, 6.1 × 10(-13) mol cm(-2) for anti-PSA antibody, 3.88 × 10(-13) mol cm(-2) for TNF-α and 7.0 × 10(-13) mol cm(-2) for PSA. Furthermore we performed antibody-antigen binding experiments and determined the specific binding ratios. The maximum possible ratio of 2 was obtained at bulk concentrations of the antigen in the μg ml(-1) range for TNF-α and PSA.

Electrical detection of dsDNA and polymerase chain reaction amplification

Food-borne pathogens and food safety-related outbreaks have come to the forefront over recent years. Estimates on the annual cost of sicknesses, hospitalizations, and deaths run into the billions of dollars. There is a large body of research on detection of food-borne pathogens; however, the widely accepted current systems are limited by costly reagents, lengthy time to completion, and expensive equipment. Our aim is to develop a label-free method for determining a change in DNA concentration after a PCR assay. We first used impedance spectroscopy to characterize the change in concentration of purified DNA in deionized water within a microfluidic biochip. To adequately measure the change in DNA concentration in PCR solution, it was necessary to go through a purification and precipitation step to minimize the effects of primers, PCR reagents, and excess salts. It was then shown that the purification and precipitation of the fully amplified PCR reaction showed results similar to the control tests performed with DNA in deionized water. We believe that this work has brought label free electrical biosensors for PCR amplification one step closer to reality.

Selective patterning of Si-based biosensor surfaces using isotropic silicon etchants

Ultra-sensitive, label-free biosensors have the potential to have a tremendous impact on fields like medical diagnostics. For the majority of these Si-based integrated devices, it is necessary to functionalize the surface with a targeting ligand in order to perform specific biodetection. To do this, silane coupling agents are commonly used to immobilize the targeting ligand. However, this method typically results in the bioconjugation of the entire device surface, which is undesirable. To compensate for this effect, researchers have developed complex blocking strategies that result in selective patterning of the sensor surface. Recently, silane coupling agents were used to attach biomolecules to the surface of silica toroidal biosensors integrated on a silicon wafer. Interestingly, only the silica biosensor surface was conjugated. Here, we hypothesize why this selective patterning occurred. Specifically, the silicon etchant (xenon difluoride), which is used in the fabrication of the biosensor, appears to reduce the efficiency of the silane coupling attachment to the underlying silicon wafer. These results will enable future researchers to more easily control the bioconjugation of their sensor surfaces, thus improving biosensor device performance.

Nanoporous waveguide sensor with optimized nanoarchitectures for highly sensitive label-free biosensing

Label-free optical biosensors have attracted much attention, and nanoporous metal-oxide membranes with uniform pore structure and diameter are promising candidates for platforms of label-free optical biosensors. However, development of such sensors with high sensitivity still remains challenging. In this paper, we report on the remarkably enhanced sensitivity of a label-free nanoporous optical waveguide (NPWG) sensor composed of a porous anodic alumina (PAA) waveguiding film and an aluminum cladding film. The enhanced sensitivity was achieved by engineering nanostructures and tuning optical properties of the PAA film. Careful tuning of the porosity, pore density, thickness, and refractive index of the PAA film could significantly improve the sensitivity of the NPWG sensor toward adsorption of bovine serum albumin (BSA) onto the PAA surface, and the optimized sensor responded to the adsorption of BSA with an extraordinarily large red shift (>300 nm) of a waveguide mode due to the large adsorption capacity of the PAA film and the inherently high sensitivity of the waveguide mode. The Fresnel calculations suggested that the potential sensitivity of the NPWG sensor was much higher than that of the conventional surface plasmon resonance (SPR) sensors.

Single nanoparticle detectors for biological applications

Nanoparticle research has become increasingly important in the context of bioscience and biotechnology. Practical use of nanoparticles in biology has significantly advanced our understanding about biological processes in the nanoscale as well as led to many novel diagnostic and therapeutic applications. Besides, synthetic and natural nanoparticles are of concern for their potential adverse effect on human health. Development of novel detection and characterization tools for nanoparticles will impact a broad range of disciplines in biological research from nanomedicine to nanotoxicology. In this article, we discuss the recent progress and future directions in the area of single nanoparticle detectors with an emphasis on their biological applications. A brief critical overview of electrical and mechanical detection techniques is given and a more in-depth discussion of label-free optical detection techniques is presented.

DNA transport and delivery in thermal gradients near optofluidic resonators

Heat generation and its impact on DNA transport in the vicinity of an optofluidic silicon photonic crystal resonator are studied theoretically and experimentally. The temperature rise is measured to be as high as 57 K for 10 mW of input power. The resulting optical trapping and biomolecular sensing properties of these devices are shown to be strongly affected by the combination of buoyancy driven flow and thermophoresis. Specifically, the region around the electromagnetic hot spot is depleted in biomolecules because of a high free energy barrier.

Modeling of coupled-resonator optical waveguide (CROW) based refractive index sensors using pixelized spatial detection at a single wavelength

We model and analyze coupled-resonator optical waveguide (CROW) based refractive index (RI) sensors using pixelized spatial detection. Our modeled cascaded Fabry-Perot (FP) CROWs reveal that the intra-band states mode-field distributions vary upon effective RI change at a single wavelength. The spatial Fourier transform of the CROW mode-field distributions, with each cavity field intensity integrated as a pixel, shows spatial frequency peak shift, which constitutes the basis of such a spatial domain sensor. The spatial domain sensing performance depends on the cavity number, the cavity length and the inter-cavity coupling. Our modeled 21-element CROW sensor attains a detection limit of 10(-4) refractive index unit (RIU) with a sensing dynamic range of 10(-3) RIU. Detailed analysis of the spatial frequency harmonic peak amplitude variation further suggests an improved detection limit. Finite-difference time-domain (FDTD) simulations of an 11-element microring CROW device shows sensitivity consistent with the FP modeling.

Low-loss silica-on-silicon waveguides

Waveguiding structures are one of the fundamental components of integrated photonic circuitry. Devices with low loss and a linear response across a wide wavelength range are especially desirable. In the present Letter, we have successfully developed and characterized low-loss silica waveguides integrated on a silicon substrate with a novel suspended cylinder geometry. The unique design creates a device that is effectively air clad, resulting in a large refractive index contrast for improved optical field confinement. The measured loss is constant from 658 to 1550 nm, and it is independent of the polarization of the input light and the input power.

Determination of binding kinetics using whispering gallery mode microcavities

Silica optical microcavity sensors show great promise in the kinetic evaluation of binding pairs, fundamental in understanding biomolecular interactions. Here, we develop and demonstrate a novel platform, based on bioconjugated silica microsphere resonators, to study the binding kinetics of the biotin-streptavidin system. We characterize the optical performance, verify the covalent attachment of biotin to the surface, and perform streptavidin detection experiments. We perform preliminary kinetic analysis of the detection data which shows the potential of whispering gallery mode resonators in the determination of the dissociation constant of the binding pair, which is in good agreement with previously published values.

Studying polymer thin films with hybrid optical microcavities

Organic and inorganic polymeric thin films have numerous applications, including solar cells, biodetection, and nanocomposites. Improving our understanding of the fundamental material behavior is critical to designing polymers with ideal behavior and increased lifetime. However, there are limited nondestructive characterization methods that are able to perform these high-resolution measurements. In this Letter, we demonstrate a method that is able to detect temperature-induced changes in the refractive index of polystyrene polymer thin films as small as 10(-7). This approach is based on optical microcavity resonators. The experimental results agree well with the theoretical simulations.

Excitation of Cy5 in self-assembled lipid bilayers using optical microresonators

Due to their sensitivity and temporal response, optical microresonators are used extensively in the biosensor arena, particularly in the development of label-free diagnostics and measurement of protein kinetics. In the present letter, we investigate using microcavities to probe molecules within biomimetic membranes. Specifically, a method for self-assembling lipid bilayers on spherical microresonators is developed and the bilayer-nature is verified. Subsequently, the microcavity is used to excite a Cy5-conjugated lipid located within the bilayer while the optical performance of the microcavity is characterized. The emission wavelength of the dye and the optical behavior of the microcavity agree with theoretical predictions.

An algorithm for three-dimensional Monte-Carlo simulation of charge distribution at biofunctionalized surfaces

In this work, a Monte-Carlo algorithm in the constant-voltage ensemble for the calculation of 3d charge concentrations at charged surfaces functionalized with biomolecules is presented. The motivation for this work is the theoretical understanding of biofunctionalized surfaces in nanowire field-effect biosensors (BioFETs). This work provides the simulation capability for the boundary layer that is crucial in the detection mechanism of these sensors; slight changes in the charge concentration in the boundary layer upon binding of analyte molecules modulate the conductance of nanowire transducers. The simulation of biofunctionalized surfaces poses special requirements on the Monte-Carlo simulations and these are addressed by the algorithm. The constant-voltage ensemble enables us to include the right boundary conditions; the dna strands can be rotated with respect to the surface; and several molecules can be placed in a single simulation box to achieve good statistics in the case of low ionic concentrations relevant in experiments. Simulation results are presented for the leading example of surfaces functionalized with pna and with single- and double-stranded dna in a sodium-chloride electrolyte. These quantitative results make it possible to quantify the screening of the biomolecule charge due to the counter-ions around the biomolecules and the electrical double layer. The resulting concentration profiles show a three-layer structure and non-trivial interactions between the electric double layer and the counter-ions. The numerical results are also important as a reference for the development of simpler screening models.

Recycling microcavity optical biosensors

Optical biosensors have tremendous potential for commercial applications in medical diagnostics, environmental monitoring, and food safety evaluation. In these applications, sensor reuse is desirable to reduce costs. To achieve this, harsh, wet chemistry treatments are required to remove surface chemistry from the sensor, typically resulting in reduced sensor performance and increased noise due to recognition moiety and optical transducer degradation. In the present work, we suggest an alternative, dry-chemistry method, based on O₂ plasma treatment. This approach is compatible with typical fabrication of substrate-based optical transducers. This treatment completely removes the recognition moiety, allowing the transducer surface to be refreshed with new recognition elements and thus enabling the sensor to be recycled.

Biosensor diagnosis of urinary tract infections: a path to better treatment?

Urinary tract infection (UTI) is among the most common bacterial infections and poses a significant healthcare burden. The standard culture-based diagnosis of UTI has a typical delay of two to three days. In the absence of definitive microbiological diagnosis at the point of care, physicians frequently initiate empirical broad-spectrum antibiotic treatment, and this has contributed to the emergence of resistant pathogens. Biosensors are emerging as a powerful diagnostic platform for infectious diseases. Paralleling how blood glucose sensors revolutionized the management of diabetes, and how pregnancy tests are now conducted in the home, biosensors are poised to improve UTI diagnosis significantly. Biosensors are amenable to integration with microfluidic technology for point-of-care (POC) applications. This review focuses on promising biosensor technology for UTI diagnosis, including pathogen identification and antimicrobial susceptibility testing, and hurdles to be surpassed in the translation of biosensor technology from bench to bedside.

Optical microspherical resonators for biomedical sensing

Optical resonators play an ubiquitous role in modern optics. A particular class of optical resonators is constituted by spherical dielectric structures, where optical rays are total internal reflected. Due to minimal reflection losses and to potentially very low material absorption, these guided modes, known as whispering gallery modes, can confer the resonator an exceptionally high quality factor Q, leading to high energy density, narrow resonant-wavelength lines and a lengthy cavity ringdown. These attractive characteristics make these miniaturized optical resonators especially suited as laser cavities and resonant filters, but also as very sensitive sensors. First, a brief analysis is presented of the characteristics of microspherical resonators, of their fabrication methods, and of the light coupling techniques. Then, we attempt to overview some of the recent advances in the development of microspherical biosensors, underlining a number of important applications in the biomedical field.

Bioconjugation strategies for microtoroidal optical resonators

The development of label-free biosensors with high sensitivity and specificity is of significant interest for medical diagnostics and environmental monitoring, where rapid and real-time detection of antigens, bacteria, viruses, etc., is necessary. Optical resonant devices, which have very high sensitivity resulting from their low optical loss, are uniquely suited to sensing applications. However, previous research efforts in this area have focused on the development of the sensor itself. While device sensitivity is an important feature of a sensor, specificity is an equally, if not more, important performance parameter. Therefore, it is crucial to develop a covalent surface functionalization process, which also maintains the device’s sensing capabilities or optical qualities. Here, we demonstrate a facile method to impart specificity to optical microcavities, without adversely impacting their optical performance. In this approach, we selectively functionalize the surface of the silica microtoroids with biotin, using amine-terminated silane coupling agents as linkers. The surface chemistry of these devices is demonstrated using X-ray photoelectron spectroscopy, and fluorescent and optical microscopy. The quality factors of the surface functionalized devices are also characterized to determine the impact of the chemistry methods on the device sensitivity. The resulting devices show uniform surface coverage, with no microstructural damage. This work represents one of the first examples of non-physisorption-based bioconjugation of microtoroidal optical resonators.

Optical ring resonators for biochemical and chemical sensing

In the past few years optical ring resonators have emerged as a new sensing technology for highly sensitive detection of analytes in liquid or gas. This article introduces the ring resonator sensing principle, describes various ring resonator sensor designs, reviews the current state of the field, and presents an outlook of possible applications and related research and development directions.