Local Refractive Index Change Research Articles

Light sheet microscope scanning of biointegrated microlasers for localized refractive index sensing

Whispering gallery mode (WGM) microlasers are highly sensitive to localized refractive index changes allowing to link their emission spectrum to various chemical, mechanical, or physical stimuli. Microlasers recently found applications in biological studies within single cells, in three-dimensional samples such as multicellular spheroids, or in vivo. However, detailed studies of biological samples also need to account for the structural heterogeneity of tissues and live animals, therefore requiring a combination of high-resolution microscopy and laser spectroscopy. Here, we design and construct a light sheet fluorescence microscope with a coupled spectrometer for use in microlaser studies for combined high-resolution, high-speed imaging and WGM spectral analysis. The light sheet illumination profile and the decoupled geometry of excitation and emission hereby directly affect the lasing and sensing properties, mainly through geometric constraints and by light coupling effects. We demonstrate the basic working principle of microlaser spectroscopy under light sheet excitation and measure the absolute refractive index within agarose and in zebrafish tail muscle tissue. We further analyze the light coupling conditions that lead to the occurrence of two separate oscillation planes. These so-called cross modes can be scanned around the entire microlaser surface, which allows to estimate a surface-averaged refractive index profile of the microlaser environment.

An advanced optic-fiber differential sensing system enhanced by molecularly imprinted polymer for specific sodium benzoate detection

A molecularly imprinted polymer (MIP) based microfiber differential demodulation sensing system for sodium benzoate (SB) concentration detection is proposed. The specific binding of MIP on the surface of microfibers with SB can lead to changes in local refractive index (RI). RI change induces a drift in the interference wavelength, which can be monitored by the power difference between two fiber Bragg gratings (FBGs). The sensing system can detect SB in the concentration range of 0.1–50 μg/ml, and interference wavelength and FBG power difference sensitivities are 0.55 nm/(μg/ml) and 2.64 dB/(μg/ml) in the low concentration range of 0.1–1 μg/ml, respectively, with a limit of detection (LOD) of 0.1 μg/ml. This microfiber differential demodulation sensing system is not only simple to fabricate, but also simplifies the demodulation equipment to reduce the cost, which providing a simple, reliable and low-cost technique for the quantitative detection of SB concentration in beverages and flavoured foods.

High-Sensitivity Detection of Changes in Local Refractive Index and Absorption by Analyzing WGM Microlaser Emission via a 2D Dispersion Spectrometer.

Microlasers have been widely used in biosensing applications because of their high sensitivity to changes in local conditions. However, in most applications, the sensitivity limit is not dictated by the microlaser line width but rather by the much worse spectral resolution of the detection system, typically a grating spectrometer. To address this issue, we built and characterized a two-dimensional (2D) dispersion spectrometer with a virtually imaged phase array etalon and a diffraction grating. The spectrometer can analyze microlaser emission with a spectral resolution of better than 0.300 pm, which enables high-precision measurements of spectral shifts in laser peak emission wavelength and sufficient resolution to detect changes in peak line width. Using commercial fluorescent microspheres as the microlasers, the 2D dispersion spectrometer demonstrated a detection limit for the refractive index change of a liquid medium of 1.37 × 10-5 RIU and a detection limit for absorption changes of less than 0.02 cm-1.

Refractive Index-Modulated LSPR Sensing in 20–120 nm Gold and Silver Nanoparticles: A Simulation Study

Localized surface plasmon resonance (LSPR) based sensing has been a simple and cost-effective way to measure local refractive index changes. LSPR materials exhibit fascinating properties that have significant implications for various bio/chemical sensing applications. In many of these applications, the focus has traditionally been on analyzing the intensity of the reflected or transmitted signals in terms of the refractive index of the surrounding medium. However, limited simulation work is conducted on investigating the refractive index sensitivity of LSPR materials. Within this context, here we simulate the refractive index sensing properties of spherical gold (Au) and silver (Ag) nanoparticles ranging from 20–120 nm diameter within 1.0 to 1.50 refractive index units (RIU). After analyzing the peak optical efficiency and peak wavelength, we report the sensing performance of these materials in terms of sensitivity, linearity and material efficiency, which we refer to as the figure of merit (FOM). Overall, our observations have revealed greatest FOM values for the smallest sized nanoparticles, a FOM of 6.6 for 20 nm AuNPs and 11.9 for 20 nm AgNPs with refractive index of 1.

Active Control of Energy Transfer in Plasmonic Nanorod-Polyaniline Hybrids.

The hybridization of plasmonic energy and charge donors with polymeric acceptors is a possible means to overcome fast internal relaxation that limits potential photocatalytic applications for plasmonic nanomaterials. Polyaniline (PANI) readily hybridizes onto gold nanorods (AuNRs) and has been used for the sensitive monitoring of local refractive index changes. Here, we use single-particle spectroscopy to quantify a previously unreported plasmon damping mechanism in AuNR-PANI hybrids while actively tuning the PANI chemical structure. By eliminating contributions from heterogeneous line width broadening and refractive index changes, we identify efficient resonance energy transfer (RET) between AuNRs and PANI. We find that RET dominates the optical response in our AuNR-PANI hybrids during the dynamic tuning of the spectral overlap of the AuNR donor and PANI acceptor. Harnessing RET between plasmonic nanomaterials and an affordable and processable polymer such as PANI offers an alternate mechanism toward efficient photocatalysis with plasmonic nanoparticle antennas.

Dual-Function Meta-Grating Based on Tunable Fano Resonance for Reflective Filter and Sensor Applications.

Localized surface plasmon resonance (LSPR)-based sensors exhibit enormous potential in the areas of medical diagnosis, food safety regulation and environmental monitoring. However, the broadband spectral lineshape of LSPR hampers the observation of wavelength shifts in sensing processes, thus preventing its widespread applications in sensors. Here, we describe an improved plasmonic sensor based on Fano resonances between LSPR and the Rayleigh anomaly (RA) in a metal-insulator-metal (MIM) meta-grating, which is composed of silver nanoshell array, an isolation grating mask and a continuous gold film. The MIM configuration offers more freedom to control the optical properties of LSPR, RA and the Fano resonance between them. Strong couplings between LSPR and RA formed a series of narrowband reflection peaks (with a linewidth of ~20 nm in full width at half maximum (FWHM) and a reflectivity nearing 100%) within an LSPR-based broadband extinction window in the experiment, making the meta-grating promising for applications of high-efficiency reflective filters. A Fano resonance that is well optimized between LSPR and RA by carefully adjusting the angles of incident light can switch such a nano-device to an improved biological/chemical sensor with a figure of merit (FOM) larger than 57 and capability of detecting the local refractive index changes caused by the bonding of target molecules on the surface of the nano-device. The figure of merit of the hybrid sensor in the detection of target molecules is 6 and 15 times higher than that of the simple RA- and LSPR-based sensors, respectively.

Tilted fiber Bragg grating surface plasmon resonance based optical fiber cadmium ion trace detection

A plasmonic tilted fiber Bragg grating (TFBG)-based sensor for the detection of cadmium ion (Cd2+) is proposed and demonstrated experimentally. The TFBG is coated with a thin gold film, where glutathione (GSH) can self-assemble on the gold coating to bond Cd2+ and form complexes. To enhance Cd2+ sensitivity, GSH-modified gold nanoparticles are used to amplify localized refractive index changes around TFBG surface, induced by Cd2+ concentration variations. The experimental results indicate a limit of detection of ∼0.06 nM, which is two orders better than reported fiber grating-based Cd2+ sensors. Additionally, the proposed TFBG sensor performs a wide dynamic range from 0.01 nM to 1000 nM for Cd2+ detections and temperature-compensated wavelength monitoring with Bragg resonance.

Emission or scattering? Discriminating the origin of responsiveness in AIEgen‐doped smart polymers using the TPE dye

AbstractAggregation‐induced emission (AIE) luminogens are attractive dyes to probe polymer properties that depend on changes in chain mobility and free volume. When embedded in polymers the restriction of intramolecular motion (RIM) can lead to their photoluminescence quantum yield (PLQY) strong enhancement if local microviscosity increases (lowering of chain mobility and free volume). Nonetheless, measuring PLQY during stimuli, i.e. heat or mechanical stress, is technically challenging; thus, emission intensity is commonly used instead, assuming its direct correlation with the PLQY. Here, by using fluorescence lifetime as an absolute fluorescence parameter, it is demonstrated that this assumption can be invalid in many commonly encountered conditions. To this aim, different polymers are loaded with tetraphenylenethylene (TPE) and characterized during the application of thermal and mechanical stress and physical aging. Under these conditions, polymer matrix transparency variation is observed, possibly due to local changes in refractive index and to the formation of microfractures. By combining different characterization techniques, it is proved that scattering can affect the apparent emission intensity, while lifetime measurements can be used to ascertain whether the observed phenomenon is due to modifications of the photophysical properties of AIE dyes (RIM effect) or to alterations in the matrix optical properties.

Mid-IR parts-per-billion level nitric oxide detection using solid-state laser intracavity photothermal sensor (SLIPS)

Highly sensitive, laser-based sensors often utilize extended gas interaction paths (e.g., multi-pass cells) that impact their size, weight, and susceptibility to misalignment, limiting their use to laboratory applications. This study reports the first demonstration of photothermal nitric oxide (NO) detection in the mid-infrared range at 5.26 µm utilizing a unique and miniature Solid-state Laser Intracavity Photothermal Sensor (SLIPS) with an ultra-short laser-gas interaction path length of 1 mm. The gas sample detection is realized inside the resonator of a monolithic diode-pumped solid-state laser (DPSSL) emitting at 1064 nm. Modulated mid-infrared radiation absorbed by NO inside the laser cavity causes local changes in photothermally-induced gas refractive index (RI). This effect results in the DPSSL frequency shifts proportional to gas concentration, which are being detected. A minimum detection limit of 50 ppbv (parts per billion of volume) and a noise equivalent absorption (NEA) coefficient of 7.9 × 10−7 cm−1 for 10 s integration time have been achieved in an ultra-compact sensing volume (4 µl) and ultra-short interaction length of the sensor as a result of its superb RI sensitivity of 1.8 × 10−11. Furthermore, the SLIPS is not limited in gas excitation laser wavelength because only a near-infrared detector is needed for spectroscopic signal readout.

Label-Free Bound-States-in-the-Continuum Biosensors.

Bound states in the continuum (BICs) have attracted considerable attentions for biological and chemical sensing due to their infinite quality (Q)-factors in theory. Such high-Q devices with enhanced light-matter interaction ability are very sensitive to the local refractive index changes, opening a new horizon for advanced biosensing. In this review, we focus on the latest developments of label-free optical biosensors governed by BICs. These BICs biosensors are summarized from the perspective of constituent materials (i.e., dielectric, metal, and hybrid) and structures (i.e., grating, metasurfaces, and photonic crystals). Finally, the current challenges are discussed and an outlook is also presented for BICs inspired biosensors.

Optoelectrode based on conductive fiber Fabry-Perot probe for simultaneous electrochemical and optical sensing

In this paper, an integrated optical fiber optoelectrode based on Fabry–Perot (F-P) cavity functionalized by ITO is first proposed to simultaneously realize the function of electrochemical working electrode and the function of fiber sensing. In the experimental prototype, the redox reaction of K3[Fe(CN)6] system in electrochemical reaction was token as an example. Simultaneous monitoring of the electrochemical and optical responses of the optoelectrode at different scan rates is discussed. The results show that the electrochemical behavior of K3[Fe(CN)6] and K4[Fe(CN)6] on the surface of the F-P-ITO optoelectrode can be successfully monitored. As the electrochemical process proceeds, local refractive index changes around the optical fiber F-P-ITO optoelectrode dominated by diffusion control will induce an obvious shift in the interference spectrum. Significantly, this novel structure of integrated F-P optoelectrode functionalized by conductive ITO based on fiber interferometer has great potentials in the fields of electrochemistry systems and biomedical applications.

(INVITED)Direct laser writing of visible and near infrared 3D luminescence patterns in glass

We report the fluorescent properties of 3D localized structure with size near the diffraction limit induced by femtosecond direct laser writing (DLW) in Yb3+ and silver-containing phosphate glass. The homogenous dispersion of the silver ions and Yb3+ ions in the glass matrix before DLW was evidenced using photo-luminescent spectroscopy and time-resolved spectroscopy. Using high repetition rate femtosecond DLW, the inscription of 3D visible and near-infrared fluorescent patterns formed by co-localization of silver cluster and Yb3+ ions was demonstrated. The local refractive index change associated with the formation of silver clusters is dependent on the laser irradiance. Confocal micro-luminescent spectroscopy for excitation wavelength in the visible range shows efficient emission of Yb3+ only on DLW induced 3D fluorescent patterns. This finding demonstrated the ability to perform thanks to DLW laser a resonant efficient nonradiative energy transfer from silver clusters to Yb3+ and allows 3D writing of near-infrared luminescence.

Digital holographic imaging of thermal signatures and its use in inhomogeneity identification

Thermal diffusion results in spatiotemporally evolving temperature distribution in material mediums. In dielectric mediums it will also lead to change in local refractive index. Digital holographic interferometry can be used for high contrast quantitative phase imaging of spatially and temporally evolving refractive index variations in dielectric mediums, arising due to temperature changes. Phase retrieved from numerically processed digital holograms recorded at different time instances can be compared to obtain this information. Since the thermal conductivity varies for materials, the thermal diffusion and hence the refractive index across them will also vary. Time varying optical path length distributions of the test sample under thermal stress, obtained from the phase maps retrieved from the recorded holograms, can be used to obtain this information. This leads to detection of regions having different thermal conductivities and hence identification of inhomogeneities in the test sample. Lens less Fourier transform digital holographic interferometry is a perfect tool to quantify optical path length distributions with nanometre accuracy to quantify spatiotemporally evolving refractive index distributions. In this technique, a single Fourier transform of the recorded hologram yields the spatial distribution of object phase, making it ideal for real time imaging and applications. Here we describe our efforts in the development lens less Fourier transform digital holographic interferometric technique for the imaging and quantification of spatiotemporally evolving refractive index distributions in test samples under thermal stress and its application in detection of surface and sub-surface inhomogeneities.

Endorsing a Hidden Plasmonic Mode for Enhancement of LSPR Sensing Performance in Evolved Metal-insulator Geometry Using an Unsupervised Machine Learning Algorithm.

Large-area nanoplasmonic structures with pillared metal-insulator geometry, also called nanomushrooms (NM), consist of an active spherical-shaped plasmonic material such as gold as its cap and silicon dioxide as its stem. NM is a geometry which evolves from its precursor, nanoislands (NI) consisting of aforementioned spherical structures on flat silicon dioxide substrates, via selective physical or chemical etching of the silicon dioxide. The NM geometry is well-known to provide enhanced localized surface plasmon resonance (LSPR) sensitivity in biosensing applications as compared to NI. However, precise optical phenomenon behind this enhancement is unknown and often associated with the existence of electric fields in the large fraction of the spatial region between the pillars of NM, usually accessible by the biomolecules. Here, we uncover the association of LSPR enhancement in such geometries with a hidden plasmonic mode by conducting magneto-optics measurements and by deconvoluting the absorbance spectra obtained during the local refractive index change of the NM and NI geometries. By the virtue of principal component analysis, an unsupervised machine learning technique, we observe an explicit relationship between the deconvoluted modes of LSPR, the differential absorption of left and right circular polarized light, and the refractive index sensitivity of the LSPR sensor. Our findings may lead to the development of new approaches to extract unknown properties of plasmonic materials or establish new fundamental relationships between less understood photonic properties of nanomaterials.

Femtosecond Laser Densification of Hydrogels to Generate Customized Volume Diffractive Gratings.

Inspired by nature’s ability to shape soft biological materials to exhibit a range of optical functionalities, we report femtosecond (fs) laser-induced densification as a new method to generate volume or subsurface diffractive gratings within ordinary hydrogel materials. We characterize the processing range in terms of fs laser power, speed, and penetration depths for achieving densification within poly(ethylene glycol) diacrylate (PEGDA) hydrogel and characterize the associated change in local refractive index (RI). The RI change facilitates the fabrication of custom volume gratings (parallel line, grid, square, and ring gratings) within PEGDA. To demonstrate this method’s broad applicability, fs laser densification was used to generate line gratings within the phenylboronic acid (PBA) hydrogel, which is known to be responsive to changes in pH. In the future, this technique can be used to convert ordinary hydrogels into multicomponent biophotonic systems.

Dual-mode independent detection of pressure and refractive index by miniature grating-coupled surface plasmon sensor.

Multiple parameters need to be monitored to analyze the kinetics of biological progresses. Surface plasmon polariton resonance sensors offer a non-invasive approach to continuously detect the local change of refractive index of molecules with high sensitivity. However, the fabrication of miniaturized, compact, and low-cost sensors is still challenging. In this paper, we propose and demonstrate a grating-coupled SPR sensor platform featuring dual mode operation for simultaneous sensing of pressure and refractive index, which can be fabricated using a highly-efficient low-cost method, allowing large-scale production. Both sensing functionalities are realized by optical means via monitoring the spectral positions of a surface plasmon polariton mode (for refractive index sensing) and Fabry-Perot or metal-insulator-metal modes (for pressure sensing), which are supported by the structure. Simultaneous measurement of refractive index with the sensitivity of 494 nm/RIU and pressure was demonstrated experimentally. The proposed platform is promising for biomonitoring that requires both high refractive index sensitivity and local pressure detection.

Single-particle spectroscopic investigation on the scattering spectrum of Au@MoS2 core-shell nanosphere heterostructure.

Owing to the uniform shape of the nanospheres, the Au@MoS2 core-shell nanosphere heterostructure enables us to design nano-optoelectronic devices and nanosensors with highly tunable and reproducible optical properties. However, until now, at the single-particle level, there is still uncertainty as to how much the scattering characteristics depend on the particle size and the local environment. In this letter, we performed an in situ single-particle study of the scattering spectrum of the Au@MoS2 core-shell nanosphere heterostructure before and after coating with the MoS2 layer. Single-particle characterization confirms that the classic quasi-static approximation (QSA) theory can be used to predict the scattering spectra of Au@MoS2 core-shell nanoparticles. Moreover, we have found that the A and B-exciton absorption peaks do not rely on the local refractive index change, while the position of the particle plasmon resonances does. Such features can be used as an internal reference for sensing applications against measurement errors, such as defocusing the imaging. Our results show that Au@MoS2 core-shell nanoparticles have the potential to become one of the promising nanosensors in the field of single-particle sensing.

In-situ DNA detection with an interferometric-type optical sensor based on tapered exposed core microstructured optical fiber

A label-free DNA biosensor based on exposed core microstructured optical fiber for in-situ real-time DNA detection has been presented and experimentally demonstrated. The sensor is fabricated by splicing a section of tapered exposed core fiber (ECF) between two single-mode fibers (SMFs), forming a multimode Mach-Zehnder interferometer (MZI). The ECF design provides the evanescent field with the sensitivity of a micro/nano optical fiber. In this paper, the ECF has a large cladding diameter (160 µm) but a small core (9 µm), and the core of the ECF is further reduced by tapering, which significantly improves the refractive index (RI) sensitivity. The sensor can detect local RI changes that occur on the surface of the optical fiber due to the binding of biomolecules. We immobilized probe DNA (pDNA) on the exposed side of the core to detect the complementary DNA (cDNA), demonstrating use for specific and label-free sensing of DNA hybridization. Experimental results show that the sensor can qualitatively detect cDNA with the sensitivity of 0.0618 nm/nM and a detection limit of 0.31 nM at a temperature of 25 °C. The proposed DNA biosensor has potential applications in fast developing fields such as medical diagnostics, cancer screening, drug testing, and environmental engineering.

In Situ Temperature-Compensated DNA Hybridization Detection Using a Dual-Channel Optical Fiber Sensor

A multifunction, high-sensitivity, and temperature-compensated optical fiber DNA hybridization sensor combining surface plasmon resonance (SPR) and Mach-Zehnder interference (MZI) has been designed and implemented. We demonstrate, for the first time to our knowledge, the dual-parameter measurement of temperature and refractive index (RI) by simultaneously using SPR and MZI in a simple single-mode fiber (SMF)-no-core fiber (NCF)-SMF structure. The experimental results show RI sensitivities of 930 and 1899 nm/RIU and temperature sensitivities of 0.4 and -1.4 nm/°C for the MZI and SPR, respectively. We demonstrate a sensitivity matrix used to simultaneously detect both parameters, solving the problem of temperature interference of RI variation-based biosensors. In addition, the sensor can also distinguish biological binding events by detecting the localized RI changes at the fiber's surface. We realize label-free sensing of DNA hybridization detection by immobilizing probe DNA (pDNA) onto the fiber as the probe to capture complementary DNA (cDNA). The experimental results show that the sensor can qualitatively detect cDNA after temperature compensation, and the limit of detection (LOD) of the sensor reaches 80 nM. The proposed sensor has advantages of high sensitivity, real time, low cost, temperature compensation, and low detection limit and is suitable for in situ monitoring, high-precision sensing of DNA molecules, and other related fields, such as gene diagnosis, kinship judgment, environmental monitoring, and so on.

Portable Nanoplasmonic Sensor for Real-Time SARS-CoV-2 RNA Detection

Introduction Fast, accurate, and in-situ environmental detection of SARS-CoV-2, the virus which causes COVID-19, is currently an unmet challenge. The gold standard detection method, polymerase chain reaction (PCR) is highly sensitive, but requires centralized lab infrastructure, specialized training, and is time consuming (> 3 hrs. time to result) [1].Other approaches more conducive to real-time and portable sensing are in development, including electrochemical [2] and plasmonic sensors [3]. However, improvements are still needed in device portability and robustness. Here, we introduce a new nanoplasmonic sensor configuration for SARS-CoV-2 RNA detection, which leverages commercially available surface plasmon readout instrumentation [4] for portable, real-time operation. Methods The sensor is configured as a plasmonic nanoparticle on mirror (NPoM) system, formed by a gold nanoparticle attached to a gold mirror by a peptide nucleic acid (PNA) probe complementary to the target RNA sequence (Figure 1A).The PNA capture probes are conjugated to the gold nanoparticles and then attached to the gold mirror using a two-step process [5]. Simple amine-coupling is used to attach the probes to the particle first, and then onto the activated gold mirror. Particle quantification is used to determine conjugation efficacy. The PNA probes act as a spacer layer, separating the nanoparticles from the mirror by approximately 10 nm.Upon hybridization between the PNA and RNA, the nanoparticle is pulled to within approximately 7 nm of the mirror below. The electromagnetic coupling between the nanoparticle and mirror is strongly dependent on the distance between the two and is transduced with nanometer precision by a pronounced shift in the resonance dip (Figure 1B). A change in distance of 3 nm is predicted to yield a ~40 nm shift in resonance wavelength.The NPoM system can either be interrogated in reflection mode (Figure 1B inset), or by total internal reflection with oblique illumination from the back side of the gold mirror. The latter enables integration with commercially available hardware for a portable, robust detection platform. Results and Conclusion Baseline experimental SARS-CoV-2 RNA detection has been obtained using gold nanoparticles on a glass substrate (Figure 1C). Standardized positive control material including the entire RNA genome of SARS-CoV-2 was linearly diluted to clinically relevant viral loads, and spiked into buffer for testing. The nanoparticles were functionalized with PNA capture probes complementary to a CDC-approved PCR panel sequence, and, upon RNA hybridization, a change in local refractive index was transduced by a shift in the extinction spectra peak (Figure 1C). The limit of detection of this system approached clinically relevant RNA loads (100,000 copies/mL).The NPoM geometry is expected to improve sensitivity and on-chip/portable system readout, owing to the narrower resonance width (cf. Figure 1B and 1C top). It is envisioned that this integrated platform could also be applied more broadly to the detection of DNA/RNA of other pathogens of interest including other viruses and bacterial species.