Bulk Refractive Index Changes Research Articles

Application of fiber loop ringdown spectroscopy technique for a new approach to beta-amyloid monitoring for Alzheimer Disease’s early detection

A novel fiber optic biosensor was purposed for a new approach to monitor amyloid beta protein fragment 1–42 (Aβ42) for Alzheimer’s Disease (AD) early detection. The sensor was fabricated by etching a part of fiber from single mode fiber loop in pure hydrofluoric acid solution and utilized as a Local Optical Refractometer (LOR) to monitor the change Aβ42 concentration in Artificial Cerebrospinal Fluid (ACSF). The Fiber Loop Ringdown Spectroscopy (FLRDS) technique is an ultra-sensitive measurement technique with low-cost, high sensitivity, real-time measurement, continuous measurement and portability features that was utilized with a fiber optic sensor for the first time for the detection of a biological signature in an ACSF environment. Here, the measurement is based on the total optical loss detection when specially fabricated sensor heads were immersed into ACSF solutions with and without different concentrations of Aβ42 biomarkers since the bulk refractive index change was performed. Baseline stability and the reference ring down times of the sensor head were measured in the air as 0.87% and 441.6 μs ± 3.9 μs, respectively. Afterward, the total optical loss of the system was measured when the sensor head was immersed in deionized water, ACSF solution, and ACSF solutions with Aβ42 in different concentrations. The lowest Aβ42 concentration of 2 ppm was detected by LOR. Results showed that LOR fabricated by single-mode fibers for FLRDS system design are promising candidates to be utilized as fiber optic biosensors after sensor head modification and have a high potential for early detection applications of not only AD but possibly also several fatal diseases such as diabetes and cancer.

Label-free multiplex sensing from buffer and immunoglobulin G sensing from whole blood with photonic crystal slabs using angle-tuning of an optical interference filter.

Direct detection of biomarkers from unpurified whole blood has been a challenge for label-free detection platforms, such as photonic crystal slabs (PCS). A wide range of measurement concepts for PCS exist, but exhibit technical limitations, which render them unsuitable for label-free biosensing with unfiltered whole blood. In this work, we single out the requirements for a label-free point-of-care setup based on PCS and present a wavelength selecting concept by angle tuning of an optical interference filter, which fulfills these requirements. We investigate the limit of detection (LOD) for bulk refractive index changes and obtain a value of 3.4 E-4 refractive index units (RIU). We demonstrate label-free multiplex detection for different types of immobilization entities, including aptamers, antigens, and simple proteins. For this multiplex setup we detect thrombin at a concentration of 6.3 µg/ml, antibodies of glutathione S-transferase (GST) diluted by a factor of 250, and streptavidin at a concentration of 33 µg/ml. In a first proof of principle experiment, we demonstrate the ability to detect immunoglobulins G (IgG) from unfiltered whole blood. These experiments are conducted directly in the hospital without temperature control of the photonic crystal transducer surface or the blood sample. We set the detected concentration levels into a medical frame of reference and point out possible applications.

Partially gold-coated tilted FBGs for enhanced surface biosensing.

To date, there is clear experimental evidence that gold-coated tilted fiber Bragg gratings (TFBGs) are highly sensitive plasmonic biosensors that provide temperature-compensated detection of analytes at concentrations in the picomolar range. As most optical biosensors, they bring an evanescent wave in the surrounding medium, which makes them sensitive to both surface refractive index variations (= the useful biosensing signal) and to bulk refractive index changes (= the non-useful signal for biosensing). This dual sensitivity makes them prone to drift. In this work, we study partially gold-coated TFBGs around their cross-section. These gratings present the ability to discriminate both volume and surface refractive index changes, which is interesting in biosensing to enhance the signal-to-noise ratio. The effects induced in the TFBGs transmitted amplitude spectra were analyzed for surrounding refractive index (SRI) changes in the range 1.3360-1.3370. Then, the gold film was biofunctionalized with human epidermal growth factor receptor (HER2) aptamers using thiol chemistry. The detection of HER2 proteins (a relevant cancer biomarker) at 10-9 g/mL, 10-8 g/mL and 10-6 g/mL demonstrated the advantage to identify environmental perturbations through the bare area of the TFBGs, which is left not functionalized. The non-specific drifts that could exist in samples are eliminated and a wavelength shift only related to the surface modification is obtained.

Waveguide Mach-Zehnder biosensor with laser diode pumped integrated single-mode silicon nitride organic hybrid solid-state laser

Single-mode organic solid-state lasers with direct emission into an optical waveguide are attractive candidates for cost-efficient coherent light sources employed in photonic lab-on-a-chip biosensors. Here, we present a combination of a dye-doped organic solid-state distributed feedback laser with a highly sensitive optical waveguide Mach-Zehnder interferometer on a silicon nitride photonic platform. This organic-hybrid laser allows for optical pumping with a laser diode in an alignment tolerant manner, which facilitates applications in point-of-care diagnostics. The sensitivity to bulk refractive index changes and the concentration dependent binding of streptavidin on a polyethyleneimine-biotin functionalized surface was studied to demonstrate the practicability of this cost-efficient coherent light source for optical waveguide biosensors.

Discrimination of Bulk and Surface Refractive Index Change in Plasmonic Sensors with Narrow Bandwidth Resonance Combs.

A method to enable surface plasmon resonance (SPR) sensors to discriminate between bulk and surface-localized refractive index changes is demonstrated with modified gold-coated tilted fiber Bragg grating SPR sensors (TFBG-SPR). Without this capability, all high-resolution SPR sensors should be using reference channels and strict temperature control to prevent the contamination of the desired detection of surface-localized chemical or binding events by drift of the refractive index of the medium, in which the experiment is carried out. The very fine comb of high-quality-factor resonances of a TFBG-SPR device coupled to the large differential sensitivity of some of the resonances to various perturbations is used to measure unambiguously the refractive index changes within a surface layer thinner than 25 nm from those of the bulk surrounding. The enabling modification of the conventional TFBG-SPR is a reduction of the gold coating from its optimum value near 50-30 nm: at this lower thickness, a surface plasmon wave can still be excited by a limited number of cladding mode resonances, but at the same time, the metal is thin enough to allow modes away from the SPR to tunnel across the metal and probe the bulk RI value. Measurements and simulations of the deposition of a self-assembled monolayer of 1-dodecanethiol in ethanol show that the bulk refractive index changes as small as 0.0004 can be distinguished from the formation of a 1 nm thick coating on the surface of the fiber.

Refractive index sensitivity of triangular Ag nanoplates in solution and on glass substrate

We report the synthesis of Ag triangular nanoplates and its application for the detection of bulk refractive index change resulting from low molecular weight analyte molecules. Triangular nanoplates of different edge lengths were synthesized using seed mediated approach with small modification in the earlier reported method [Adv. Funct. Mater. 18, 2005–2016 (2008)]. Increase in the edge length of nanoplates was observed with decrease in the volume of seed nanoparticle used. The synthesized nanoplates in colloidal solution were investigated for the refractive index sensing. The nanoplates synthesized with 100 μL seed volume showed maximum sensitivity for glucose and glycerol as 465 and 461 nm/RIU respectively. Triangular nanoplates with highest sensitivity in the solution were immobilized on silanized glass surface using 3-aminopropyl-trimethoxysilane as an adhesion layer. The refractive index sensitivity of immobilized nanoplates was found to be 193 and 204 nm/RIU for glucose and glycerol solutions respectively. The significant decrease in sensitivity could be due to damping of plasmon resonance on substrate or non-availability of all surfaces. The immobilized nanoplates on glass surface can be used as nanoplasmonic sensor chip for chemical and biological analyte detection.

Optimization of Au:CuO Nanocomposite Thin Films for Gas Sensing with High-Resolution Localized Surface Plasmon Resonance Spectroscopy.

Gas sensing based on bulk refractive index (RI) changes, has been a challenging task for localized surface plasmon resonance (LSPR) spectroscopy, presenting only a limited number of reports in this field. In this work, it is demonstrated that a plasmonic thin film composed of Au nanoparticles embedded in a CuO matrix can be used to detect small changes (as low as 6 × 10-5 RIU) in bulk RI of gases at room temperature, using a high-resolution LSPR spectroscopy system. To optimize the film's surface, a simple Ar plasma treatment revealed to be enough to remove the top layers of the film and to partially expose the embedded nanoparticles, and thus enhance the film's gas sensing capabilities. The treated sample exhibits high sensitivity to inert gases (Ar, N2), presenting a refractive index sensitivity (RIS) to bulk RI changes of 425 nm/RIU. Furthermore, a 2-fold signal increase is observed for O2, showing that the film is clearly more sensitive to this gas due to its oxidizing nature. The results showed that the Au:CuO thin film system is a RI sensitive platform able to detect inert gases, which can be more sensitive to detect noninert gases as O2 or even other reactive species.

Under-Etched Plasmonic Disks on Indium Tin Oxide for Enhanced Refractive Index Sensing on a Combined Electrochemical and Optical Platform.

A simple approach to enhance the refractive index sensitivity of gold nanodisks immobilized on electrically conducting indium tin oxide (ITO) substrates has been demonstrated. A two-fold increase in sensitivity to bulk refractive index change was achieved by substrate under-etching of gold nanodisks on ITO in 50 mM sulfuric acid. The influence of an intermediate titanium adhesion layer was investigated and was found to markedly influence the etching pattern and time. Etching with an adhesion layer resulted in enhanced refractive index sensitivity on disk-on-pin like structures after long etching times, whereas etching of disks deposited directly on ITO resulted in a disk-on-pincushion like configuration and similarly enhanced sensitivity already at shorter times. The gold disks remained electrically connected to the ITO substrate throughout etching and allowed site-specific electrodeposition of poly(3-aminophenol) at the nanodisks, showing enhanced thin-film refractive index sensitivity. This work demonstrates a simple method for enhancing refractive index sensitivity of nanostructures on ITO substrates for combined electrochemical and optical platforms, and subsequently a method to modify the surface of the electrically connected nanostructures, which has potential application in biosensing.

Fiber optic sensor based on ZnO nanowires decorated by Au nanoparticles for improved plasmonic biosensor

Fiber-optic-based localized surface plasmon resonance (FO-LSPR) sensors with three-dimensional (3D) nanostructures have been developed. These sensors were fabricated using zinc oxide (ZnO) nanowires and gold nanoparticles (AuNPs) for highly sensitive plasmonic biosensing. The main achievements in the development of the biosensors include: (1) an extended sensing area, (2) light trapping effect by nanowires, and (3) a simple optical system based on an optical fiber. The 3D nanostructure was fabricated by growing the ZnO nanowires on the cross-section of optical fibers using hydrothermal synthesis and via immobilization of AuNPs on the nanowires. The proposed sensor outputted a linear response according to refractive index changes. The 3D FO-LSPR sensor exhibited an enhanced localized surface plasmon resonance response of 171% for bulk refractive index changes when compared to the two-dimensional (2D) FO-LSPR sensors where the AuNPs are fixed on optical fiber as a monolayer. In addition, the prostate-specific antigen known as a useful biomarker to diagnose prostate cancer was measured with various concentrations in 2D and 3D FO-LSPR sensors, and the limits of detection (LODs) were 2.06 and 0.51 pg/ml, respectively. When compared to the 2D nanostructure, the LOD of the sensor with 3D nanostructure was increased by 404%.

Optofluidic refractive index sensor based on asymmetric diffraction.

A novel optofluidic refracrtive index (RI) sensor was proposed based on asymmetric Fraunhofer diffraction. In-plane optofluidic lens, light source, slit, diffraction pattern visualization zone and optical path were integrated into the microfluidic networks to avoid the manual alignment of the optical components as well as to reduce the cost of external bulky components. Unlike the conventional RI sensor, this device visualizes the bulk refractive index change of the liquid through a diffraction image, which is readily read-out for clinical diagnosis right at the point-of-care or on-site security check. In the experiment, the device can measure a RI change of as low as ~10-5 RIU. A low noise-equivalent detection limit (NEDL) of ~10-6 refractive index unit (RIU) and high sensitivity of ~1.1 × 104/RIU were achieved. The new device is practical and suitable to be extended for high throughput applications by simultaneously reading multiple chips with an 2D-array image sensor.

Plasmonic versus All-Dielectric Nanoantennas for Refractometric Sensing: A Direct Comparison

In comparison to nanoplasmonic structures, resonant high-index dielectric nanoantennas hold several advantages that may benefit nanophotonic applications, including CMOS compatibility and low ohmic losses. One such application area might be label-free refractometric sensing, where changes in individual antenna resonance properties are used to quantify changes in the surrounding refractive index, for example, due to biomolecular binding. Here, we analyze and compare the sensing performance of silicon and gold nanodisks using a common and unbiased testing framework. We find that the all-dielectric system is fully capable of effectively monitoring small changes in bulk refractive index and biomolecular coverage, but the sensitivity is five to ten times lower than the plasmonic counterpart. However, this drawback is partly compensated for by a more linear response to adsorbate layer thickness changes and an approximately four times smaller susceptibility to photothermal heating. Finally, dielectric sensors may show promise if certain strategies are employed to improve their performance, which could thus bridge the gap between the two systems.

Selective Uropathogenic E. coli Detection Using Crossed Surface-Relief Gratings.

Urinary tract infections (UTIs) are one of the major burdens on public healthcare worldwide. One of the primary causes of UTIs is the invasion of the urinary tract by uropathogenic Escherichia coli (UPEC). Improper treatment of bacterial infections like UTIs with broad-spectrum antibiotics has contributed to the rise of antimicrobial resistance, necessitating the development of an inexpensive, rapid and accurate detection of UPEC. Here, we present real-time, selective and label-free detection of UPEC using crossed surface-relief gratings (CSRGs) as nanometallic sensors incorporated into an optical sensing platform. CSRGs enable real-time sensing due to their unique surface plasmon resonance (SPR)-based light energy exchange, resulting in detection of a very-narrow-bandwidth SPR signal after the elimination of residual incident light. The platform’s sensing ability is experimentally demonstrated by the detection of bulk refractive index (RI) changes, with a bulk sensitivity of 382.2 nm/RIU and a resolution in the order of 10−6 RIU. We also demonstrate, for the first time, CSRG-based real-time selective capture and detection of UPEC in phosphate-buffered saline (PBS) solution, in clinically relevant concentrations, as opposed to other UTI-causing Gram-negative bacteria. The platform’s detection limit is calculated to be 105 CFU/mL (concentration on par with the clinical threshold for UTI diagnosis), with a dynamic range spanning four orders of magnitude. This work paves the way for the development of inexpensive point-of-care diagnosis devices focusing on effective treatment of UTIs, which are a burden on public healthcare due to the rise in the number of cases and their recurrences in the recent past.

Improved method for estimating adlayer thickness and bulk RI change for gold nanocrescent sensors

This paper presents a novel method employing the maximum likelihood estimation (MLE) technique alongside a nonlinear sensor response model to improve and extract more quantitative sensing results for localized surface plasmon resonance biosensors. The nonlinear response model treats the sensor response as a nonlinear function of the biomolecular adlayer thickness. This method makes use of the multiple resonance characteristic of nanocrescent structures in order to estimate the adlayer thickness and bulk refractive index (RI) change. Nanoimprint lithography is used here to fabricate the nanostructures. The finite element method (FEM) is used to model the nanocrescents and numerically validate the nonlinear-MLE method. Comparing to the established linear model, the proposed nonlinear-MLE method achieves 75% improvement in the limit of detection based on the estimated adlayer thickness and improves the bulk RI resolution by two orders of magnitude.

Liquid phantoms for near-infrared and diffuse correlation spectroscopies with tunable optical and dynamic properties.

We present the recipe and characterization for preparing liquid phantoms that are suitable for both near-infrared spectroscopy and diffuse correlation spectroscopy. The phantoms have well-defined and tunable optical and dynamic properties, and consist of a solution of water and glycerol with fat emulsion as the scattering element. The recipe takes into account the effect of bulk refractive index changes due to the addition of glycerol, which is commonly used to alter the sample viscosity.

LSPR and Interferometric Sensor Modalities Combined Using a Double-Clad Optical Fiber.

We report on characterization of an optical fiber-based multi-parameter sensor concept combining localized surface plasmon resonance (LSPR) signal and interferometric sensing using a double-clad optical fiber. The sensor consists of a micro-Fabry-Perot in the form of a hemispherical stimuli-responsive hydrogel with immobilized gold nanorods on the facet of a cleaved double-clad optical fiber. The swelling degree of the hydrogel is measured interferometrically using the single-mode inner core, while the LSPR signal is measured using the multi-mode inner cladding. The quality of the interferometric signal is comparable to previous work on hydrogel micro-Fabry-Perot sensors despite having gold nanorods immobilized in the hydrogel. We characterize the effect of hydrogel swelling and variation of bulk solution refractive index on the LSPR peak wavelength. The results show that pH-induced hydrogel swelling causes only weak redshifts of the longitudinal LSPR mode, while increased bulk refractive index using glycerol and sucrose causes large blueshifts. The redshifts are likely due to reduced plasmon coupling of the side-by-side configuration as the interparticle distance increases with increasing swelling. The blueshifts with increasing bulk refractive index are likely due to alteration of the surface electronic structure of the gold nanorods donated by the anionic polymer network and glycerol or sucrose solutions. The recombination of biotin-streptavidin on gold nanorods in hydrogel showed a 7.6 nm redshift of the longitudinal LSPR. The LSPR response of biotin-streptavidin recombination is due to the change in local refractive index (RI), which is possible to discriminate from the LSPR response due to changes in bulk RI. In spite of the large LSPR shifts due to bulk refractive index, we show, using biotin-functionalized gold nanorods binding to streptavidin, that LSPR signal from gold nanorods embedded in the anionic hydrogel can be used for label-free biosensing. These results demonstrate the utility of immobilizing gold nanorods in a hydrogel on a double-clad optical fiber-end facet to obtain multi-parameter sensing.

A Single-Point, Multiparameter, Fiber Optic Sensor Based on a Combination of Interferometry and LSPR

We demonstrate a new single-point, multiparameter, fiber optic sensor concept based on a combination of interferometric and plasmonic sensor modalities on an optical fiber end face. The sensor consists of an extrinsic Fabry–Perot interferometer in the form of a hemispherical stimuli-responsive hydrogel with immobilized gold nanoparticles (GNPs). The GNPs exhibit local surface plasmon resonance (LSPR) that is sensitive toward the local refractive index (RI) of the surrounding environment, whereas the stimuli-responsive hydrogel is sensitive toward specific chemical compounds. We evaluate the quality of the interferometric and LSPR signals as a function of GNP concentration and of hydrogel swelling degree stimulated by ethanol solutions. The GNPs have little influence on the visibility of the Fabry–Perot etalon. The swelling degree of the hydrogel, with corresponding bulk RI changes, has little influence on the local surface RI of the GNPs. We expect this novel sensor concept to be of great value for biosensors for medical applications.

Plasmonic Metaparticles on a Blackbody Create Vivid Reflective Colors for Naked-Eye Environmental and Clinical Biodetection.

Plasmonic dipoles are famous for their strong absorptivity rather than their reflectivity. Here, the as-yet unknown specular reflection and the Brewster effect of ultrafine plasmonic dipoles, metaparticles, are introduced and exploited as the basis of new design rules for advanced applications. A configuration of "Plasmonic metaparticles on a blackbody" is demonstrated and utilized for the design of a tailored perfect-colored absorber and for visual detection of environmental dielectrics that is not readily done by extinction plasmonics. Moreover, the Plasmonic Brewster Wavelength (PBW) effect is introduced as a new platform for the naked-eye and bulk biodetection of analytes. The technique operates based on slight changes of molecular polarizability which is not detectable via conventional plasmon resonance techniques. As a specific highlight, the clinical applicability of the PBW method is demonstrated while addressing the transduction plasmonic techniques' challenge in detection of bulk refractive index changes of the healthy and diseased human serum exosomes. Finally, the sputtering-based fabrication method used here is simple, inexpensive, and scalable, and does not require the sophisticated patterning approach of lithography or precise alignment of light coupling for the biodetection.

Origin of the Optical Response of a Dye-doped Plasticized Poly(vinyl chloride)-based Photonic Crystal Ion Sensor.

We investigated the origin of the optical responses of a two-dimensional photonic crystal (2D-PhC) optode membrane ion sensor based on plasticized poly(vinyl chloride) (PVC) containing lipophilic dye. We previously developed a highly sensitive potassium ion sensor. However, the contributions of various factors to the sensor's optical response could not be distinguished. Here, we synthesized a lipophilic dye with spectral changes that do not overlap with those of propagated light in the PhC. Using this synthesized dye excluded the contribution of light absorption by the dye to the peak intensity change of the ion sensor. In refractive index standard solutions, the bulk refractive index change upon ion extraction contributed negligibly. The contribution of diffracted light scattering by optical transparency change of the plasticized PVC upon ion extraction accounted for approximately 45% of the ion sensor's total response. These findings are important for the development of highly sensitive plasticized PVC-based 2D-PhC ion sensors.

Integration of core/shell nanoparticle and QCM-D sensors in a single device: A new approach to the in situ detection of solvent content in thin adsorbed films with minimized response to spurious bulk refractive index changes

A combined optical and acoustic wave based setup for the time-resolved determination of solvent content in thin adsorbed layers is presented. Implementation of the novel device is achieved by forming a core/shell nanoparticle sensor based on localized plasmon resonance directly on the surface of a quartz crystal microbalance (QCM) integrated in commercially available QCM-D instrumentation. A peculiarity of the optical sensor is the presence of a “magic angle”, under which changes of the bulk refractive index of the adjacent media do not influence the sensor response. Kinetic studies of fibrinogen adsorption with the combined setup and the optical sensor set to the magic angle show that the surface-bound films contain a considerable amount of water with a dry/wet mass ratio of about 26%. Comparison to literature values suggests that the water molecules are not only firmly bound to the proteins in form of a hydration shell but also entrapped in interstitial regions of the adsorbate. The high water content detected in the films confirms the importance of the presented approach. Direct comparison of the adsorption kinetics for dry and wet mass reveals that water entrapment occurs on a longer time scale than mere protein binding.

Interferogram Average over Wavelength (IAW) Reflectance Spectroscopy for Nanostructured Porous Silicon Refractive Index Optical (Bio)Sensors: A Novel Ultrasensitive Technique for Label-Free Biosensing

Porous silicon (PSi) is a nanostructured material increasingly exploited for both refractometric and (bio)sensing applications, though currently suffering of restricted real applications due to insufficient sensitivity and, in turn, poor limit of detection [1]. Here we report on development, characterization, and application (to both refractometry and biosensing) of a novel ultrasensitive technique for the label-free discrimination of either bulk or surface refractive index changes (namely, Interferogram Average over Wavelength – IAW –reflectance spectroscopy) using nanostructured PSi interferometer. As to refractometric applications, a minimum bulk refraction index variation of 10-6 RIU was experimentally measured using NaCl aqueous solutions, with a theoretical limit of detection of 10-8RIU. As to biosensing applications, a minimum concentration of TNFα, a protein biomarker of inflammation and sepsis, at concentration of 3.0 nM was experimentally monitored, with high selectivity and limit of detection of 200 pM. Both these results represent a 1000-fold improvement with respect to the commonly used fast Fourier Transform reflectance spectroscopy for PSi interferometers. The IAW reflectance spectroscopy relies on the calculation of the average value over wavelength of spectral interferograms, namely IAW value. Interferograms are calculated by subtraction (intensity, wavelength by wavelength) of the reflection spectrum acquired after infiltration of the target analyte within the nanopores of the PSi interferometer, from a reference reflection spectrum. This technique allows take simultaneously into account all changes occurring on the reflection spectrum of the interferometer, both in terms of intensity, phase, and frequency variation of the Fabry−Perot fringes that are originated by constructive−destructive interference of light within the interferometer. As proof-of-concept for refractometric applications, aqueous saline solutions with different NaCl concentrations in the range 0.001%−10% were infiltrated inside the nanopores of the PSi interferometer and the corresponding IAW signals calculated from experimental reflection spectra measured. Refractive index variations for the different NaCl concentrations were estimated thorough the use of Lorentz-Lorenz equation. A sigmoidal trend encompassing the whole range of NaCl concentrations tested was obtained, with a clear discrimination of a minimum bulk refractive index variation of 10-6 RIU with signal-to-noise ratio (S/N) of about 5. The results were statistically validated over 4 replica of the same experiment and are in agreements with the theoretical calculations. As a benchmark, the same experimental set of reflection spectra was used to calculate effective optical thickness (EOT) values using FFT reflection spectroscopy, which is the technique commonly empoyed for PSi interferometers. Discrimination of a minimum bulk refractive index variation of 10-3RIU was achieved using EOT values. As proof-of-concept for biosensing applications, the proposed IAW reflectance spectroscopy was used to demonstrate reliable detection of BSA unspecific adsorption in PSi interferometers at concentrations in the range from 150 pM to 15 μM, down to 3 orders of magnitude lower than those targeted in the current literature using a PSi interferometers operating in label-free mode without any amplification strategy. Good sample-to-sample reproducibility over the whole range of tested concentrations (%CV = 16%) and good signal-to-noise ratio also at the lowest tested concentration (S/N ≈ 4.6 at 150 pM) were achieved, with a detection limit (DL) of 20 pM (20 femtomoles, 1 mL) [2]. Further, the IAW reflectance spectroscopy was applied to a “real” biosensor through the development of a label-free PSi interferometric aptasensor able to specifically detect tumor necrosis factor alpha (TNFα, a protein biomarker of inflammation and sepsis). A lowest concentration down to 3.0 nM with signal-to-noise ratio (S/N) of 10.6 and DL of 200 pM were achieved. This results represent a 10000-fold improvement with respect to direct (i.e., unamplified) label-free PSi biosensors and pushes PSi biosensors close to the most sensitive optical and label-free transduction techniques, e.g., surface plasmon resonance (SPR) for which a lowest DL of 100 pM in label-free aptasensing is reported [3]. In conclusion, the IAW reflectance spectroscopy envisages bringing PSi optical (bio)sensors at the forefront of ultrasensitive label-free biosensing techniques, with application for point-of-care clinical analysis where low analyte concentrations are required to be detected in small volume of biological samples. Reference [1] Vilensky, R., Bercovici, M., Segal, E. Adv. Funct. Mater. 25, 6725–6732 (2015). [2] Mariani, S., Strambini, L. M., Barillaro, G., Anal. Chem. 88, 8502–8509 (2016). [3] Mariani, S., Pino, L., Strambini, L. M., Tedeschi, L., Barillaro, G. ACS Sensors (2016). doi:10.1021/acssensors.6b00634.