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Biosensors are indispensable tools for public, global, and personalized healthcare as they provide tests that can be used from early disease detection and treatment monitoring to preventing pandemics. We introduce single-wavelength imaging biosensors capable of reconstructing spectral shift information induced by biomarkers dynamically using an advanced data processing technique based on an optimal linear estimator. Our method achieves superior sensitivity without wavelength scanning or spectroscopy instruments. We engineered diatomic dielectric metasurfaces supporting bound states in the continuum that allows high-quality resonances with accessible near-fields by in-plane symmetry breaking. The large-area metasurface chips are configured as microarrays and integrated with microfluidics on an imaging platform for real-time detection of breast cancer extracellular vesicles encompassing exosomes. The optofluidic system has high sensing performance with nearly 70 1/RIU figure-of-merit enabling detection of on average 0.41 nanoparticle/µm 2 and real-time measurements of extracellular vesicles binding from down to 204 femtomolar solutions. Our biosensors provide the robustness of spectrometric approaches while substituting complex instrumentation with a single-wavelength light source and a complementary-metal-oxide-semiconductor camera, paving the way toward miniaturized devices for point-of-care diagnostics. The authors engineer a type of bound states in the continuum in diatomic dielectric metasurfaces, allowing for high-quality resonances with accessible enhanced fields. Metasurface microarrays are integrated with microfluidics on an imaging platform for real-time detection of biosamples, based on reconstructing spectral shift information.

This Review discusses recent research efforts to confine and guide light at the nanoscale without using metals. The ideal material for nanophotonic applications will have a large refractive index at optical frequencies, respond to both the electric and magnetic fields of light, support large optical chirality and anisotropy, confine and guide light at the nanoscale, and be able to modify the phase and amplitude of incoming radiation in a fraction of a wavelength. Artificial electromagnetic media, or metamaterials, based on metallic or polar dielectric nanostructures can provide many of these properties by coupling light to free electrons (plasmons) or phonons (phonon polaritons), respectively, but at the inevitable cost of significant energy dissipation and reduced device efficiency. Recently, however, there has been a shift in the approach to nanophotonics. Low-loss electromagnetic responses covering all four quadrants of possible permittivities and permeabilities have been achieved using completely transparent and high-refractive-index dielectric building blocks. Moreover, an emerging class of all-dielectric metamaterials consisting of anisotropic crystals has been shown to support large refractive index contrast between orthogonal polarizations of light. These advances have revived the exciting prospect of integrating exotic electromagnetic effects in practical photonic devices, to achieve, for example, ultrathin and efficient optical elements, and realize the long-standing goal of subdiffraction confinement and guiding of light without metals. In this Review, we present a broad outline of the whole range of electromagnetic effects observed using all-dielectric metamaterials: high-refractive-index nanoresonators, metasurfaces, zero-index metamaterials and anisotropic metamaterials. Finally, we discuss current challenges and future goals for the field at the intersection with quantum, thermal and silicon photonics, as well as biomimetic metasurfaces.

Direct optical detection has proven to be a highly interesting tool in biomolecular interaction analysis to be used in drug discovery, ligand/receptor interactions, environmental analysis, clinical diagnostics, screening of large data volumes in immunology, cancer therapy, or personalized medicine. In this review, the fundamental optical principles and applications are reviewed. Devices are based on concepts such as refractometry, evanescent field, waveguides modes, reflectometry, resonance and/or interference. They are realized in ring resonators; prism couplers; surface plasmon resonance; resonant mirror; Bragg grating; grating couplers; photonic crystals, Mach-Zehnder, Young, Hartman interferometers; backscattering; ellipsometry; or reflectance interferometry. The physical theories of various optical principles have already been reviewed in detail elsewhere and are therefore only cited. This review provides an overall survey on the application of these methods in direct optical biosensing. The “historical” development of the main principles is given to understand the various, and sometimes only slightly modified variations published as “new” methods or the use of a new acronym and commercialization by different companies. Improvement of optics is only one way to increase the quality of biosensors. Additional essential aspects are the surface modification of transducers, immobilization strategies, selection of recognition elements, the influence of non-specific interaction, selectivity, and sensitivity. Furthermore, papers use for reporting minimal amounts of detectable analyte terms such as value of mass, moles, grams, or mol/L which are difficult to compare. Both these essential aspects (i.e., biochemistry and the presentation of LOD values) can be discussed only in brief (but references are provided) in order to prevent the paper from becoming too long. The review will concentrate on a comparison of the optical methods, their application, and the resulting bioanalytical quality.

Background Transfer of passive immunity in calves can be assessed by direct measurement of immunoglobulin G (IgG) by methods such as radial immunodiffusion (RID) or turbidimetric immunoassay (TIA). IgG can also be measured indirectly by methods such as serum refractometry (REF) or Brix refractometry (BRIX). Objectives To determine the accuracy of REF and BRIX for assessment of inadequate transfer of passive immunity (ITPI) in calves. Design Systematic review and meta‐analysis of diagnostic accuracy studies. Methods Databases (PubMed and CAB , Searchable Proceedings of Animal Science) and Google Scholar were searched for relevant studies. Studies were eligible if the accuracy (sensitivity and specificity) of REF or BRIX was determined using direct measurement of IgG by RID or turbidimetry as the reference standard. The study population included calves <14 days old that were fed with natural colostrum (colostrum replacement products were excluded). Quality assessment was performed by the QUADAS‐2 tool. Hierarchical models were used for meta‐analysis. Results From 1,291 references identified, 13 studies of 3,788 calves were included. Of these, 11 studies evaluated REF and 5 studies evaluated BRIX. The median (range) prevalence of ITPI (defined as calves with IgG <10 g/L by RID or TIA) was 21% (1.3–56%). Risk of bias and applicability concerns were generally low or unclear. For REF, summary estimates were obtained for 2 different cutoffs: 5.2 g/dL (6 studies) and 5.5 g/dL (5 studies). For the 5.2 g/dL cutoff, the summary sensitivity (95% CI) and specificity (95% CI) were 76.1% (63.8–85.2%) and 89.3% (82.3–93.7%), and 88.2% (80.2–93.3%) and 77.9% (74.5–81.0%) for the 5.5 g/dL cutoff. Due to the low number of studies using the same cutoffs, summary estimates could not be obtained for BRIX. Conclusions and Clinical Importance Despite their widespread use on dairy farms, evidence about the optimal strategy for using refractometry, including the optimal cutoff, are sparse (especially for BRIX). When using REF to rule out ITPI in herds, the 5.5 g/dL cutoff may be used whereas for ruling in ITPI, the 5.2 g/dL cutoff may be used.

Diabetes mellitus is a global health issue affecting millions of people and requires regular glucose level monitoring. Current non-invasive methods, such as urinalysis (including colorimetry and biosensors), are primarily laboratory-based and lack user-friendliness, limiting their practicality for continuous glucose monitoring. Although promising, research on smartphone-integrated laser refractometry for glucose detection remains limited. To address this gap, a non-contact, smartphone-based laser refractometer for glucose monitoring was developed. This prototype measures the refractive index of urine by analyzing the refracted length of a laser line, which correlates with fasting blood glucose concentrations. The system uses a smartphone to capture high-resolution images of the laser line generated by total internal reflection through a rod and refraction through the urine sample. Assessments were conducted using controlled glucose concentrations, varying turbidity levels, different samples volume, and shelf-life conditions. Volumetric and shelf-life variations showed no significant impact on results, whereas turbidity assessments revealed a limitation of up to 57 nephelometric turbidity units (NTU). Fasting glucose levels measured from using the developed system were compared with laboratory fasting blood glucose results, yielding a correlation coefficients of 0.89 and a sensitivity of 4.8 mg/dL. The system is low-cost, making it accessible and suitable for telemedicine applications, offering remote glucose monitoring for patients. This approach paves the way for clinically relevant glucose detection in diabetic patients without the need for invasive finger-prick blood sampling.

This study was aimed at the assessment of the reliability and performance of the Brix refractometer for the determination of colostral IgG concentrations in sheep. A total of 190 colostrum samples were collected from 150 ewes of various breeds. Out of the colostrum samples, 101 were taken at parturition, 10 at 6 h postpartum, 6 at 8 h postpartum, 2 at 10 h postpartum, 65 at 12 h postpartum and 6 at 24 h postpartum. The Brix percentages of the colostrum samples were determined using a Brix refractometer, and the colostral IgG concentrations were measured with the radial immunodiffusion (RID) technique. Correlation coefficients were calculated between the Brix percentages and RID‐IgG values. Receiver operating characteristics (ROC) analyses were performed to determine the optimal Brix thresholds for the prediction of colostral IgG concentrations using <20, <50, <75 and <100 g/L thresholds. The misclassification cost term (MCT) analyses were performed to test the robustness of the Brix thresholds based on the relative costs of misclassifications. The mean IgG concentration and Brix percentage of the ewe colostrum samples were determined as 62.7 g/L and 27.2%, respectively. A high correlation was detected between the colostral RID‐IgG concentrations and Brix percentages (r = 0.83). Brix percentages of ≤25.0%, ≤25.9%, ≤27.5% and ≤31.9% indicated IgG concentrations of <20, <50, <75 and <100 g/L, respectively. The Brix refractometer was demonstrated to be a low‐cost, practical and reliable tool for the determination of colostrum quality in sheep. It is recommended that the four Brix thresholds determined in this study be used strategically in feeding lambs in need of colostral supplementation due to inadequate or poor‐quality colostrum production by their dams.

The refractive index and absorption coefficient of a medium in the infrared range are measured using visible spectral range components. The technique relies on nonlinear interference of infrared and visible photons, produced by down-conversion. Spectral measurements in the infrared optical range provide unique fingerprints of materials, which are useful for material analysis, environmental sensing and health diagnostics 1 . Current infrared spectroscopy techniques require the use of optical equipment suited for operation in the infrared range, components of which face challenges of inferior performance and high cost. Here, we develop a technique that allows spectral measurements in the infrared range using visible-spectral-range components. The technique is based on nonlinear interference of infrared and visible photons, produced via spontaneous parametric down conversion 2 , 3 . The intensity interference pattern for a visible photon depends on the phase of an infrared photon travelling through a medium. This allows the absorption coefficient and refractive index of the medium in the infrared range to be determined from the measurements of visible photons. The technique can substitute and/or complement conventional infrared spectroscopy and refractometry techniques, as it uses well-developed components for the visible range.

In this study, we investigate a multispectral plasmonic refractive index sensor based on a ring resonator supercell array for the detection of carcinoembryonic antigen (CEA), prostate-specific antigen (PSA), and hemoglobin (Hb). The sensor exhibits three resonance modes within the 1000–3000 nm wavelength range, analyzed using the finite-difference time-domain (FDTD) method. Through meticulous optimization of the sensor’s geometrical structure, we achieve a high figure of merit (FOM) and sensitivity. Notably, the second resonance mode attains the highest sensitivity of 913.51 nm/RIU. Furthermore, the third resonance mode achieves an FOM exceeding 70 RIU −1 , underscoring its potential for specialized biomedical applications. Extensive research has established a strong correlation between elevated antigen concentrations, including CEA, PSA, and hemoglobin, and the presence of cancer and other critical diseases. With advancements in optical biosensing technology, this sensor offers a promising pathway toward cost-effective, portable, and highly sensitive diagnostic tools, contributing to public health and improved quality of life.

Characterization of the size and material properties of particles in liquid suspensions is in very high demand, for example, in the analysis of colloidal samples or of bodily fluids such as urine or blood plasma. However, existing methods are limited in their ability to decipher the constituents of realistic samples. Here we introduce iNTA as a new method that combines interferometric detection of scattering with nanoparticle tracking analysis to reach unprecedented sensitivity and precision in determining the size and refractive index distributions of nanoparticles in suspensions. After benchmarking iNTA with samples of colloidal gold, we present its remarkable ability to resolve the constituents of various multicomponent and polydisperse samples of known origin. Furthermore, we showcase the method by elucidating the refractive index and size distributions of extracellular vesicles from Leishmania parasites and human urine. The current performance of iNTA already enables advances in several important applications, but we also discuss possible improvements. iNTA combines interferometric detection of scattering with nanoparticle tracking analysis for determining the size and refractive index distributions of nanoparticles in suspension with high sensitivity and precision.