Photonic crystal enhanced fluorescence emission and blinking suppression for single quantum dot digital resolution biosensing

Photonic crystal-assisted sub-bandgap photocatalysis via triplet-triplet annihilation upconversion for the degradation of environmental organic pollutants

This study explores novel approaches to enhance photocatalysis efficiency by introducing a photonic crystal (PC)-enhanced, multi-layered sub-bandgap photocatalytic reactor. The design aims to effectively utilize sub-bandgap photons that might otherwise go unused. The device consists of three types of layers: (1) two polymeric triplet-triplet annihilation upconversion (TTA-UC) layers converting low-energy green photons (λEx = 532 nm, 2.33 eV) to high-energy blue photons (λEm = 425 nm, 2.92 eV), (2) a platinum-decorated WO3 layer (Eg = 2.8 eV) serving as a visible-light photocatalyst, and (3) a PC layer optimizing both TTA-UC and photocatalysis. The integration of the PC layer resulted in a 1.9-fold increase in UC emission and a 7.9-fold enhancement in hydroxyl radical (•OH) generation, achieved under low-intensity sub-bandgap irradiation (17.6 mW cm-2). Consequently, the combined layered structure of TTA/Pt-WO3/TTA/PC achieved a remarkable 38.8-fold improvement in •OH production, leading to outstanding degradation capability for various organic pollutants (e.g., 4-chlorophenol, bisphenol A, and methylene blue). This multi-layered sub-bandgap photocatalytic structure, which uniquely combines TTA-UC and PC layers, offers valuable insights into designing efficient photocatalytic systems for future solar-driven environmental remediation.

Ultrasensitive Detection of Cancer Biomarkers Using Photonic-Crystal-Enhanced Single-Molecule Imaging

The rapid and sensitive quantification of low-abundance protein markers holds immense significance in early disease diagnosis and treatment. Single-molecule fluorescence imaging exhibits very high detection sensitivity and thus has great application potential in this area. The single-molecule signal, however, is often susceptible to interference from background noise due to its inherently weak intensity. A variety of signal amplification techniques based on cascading reactions have been developed to improve the signal-to-noise ratio of single-molecule imaging. Nevertheless, the operation of these methods is typically complicated and time-consuming, which limits the clinical application. Herein, we introduce an enzyme-free, photonic-crystal-based single-molecule (PC-SM) biochip for cost-effective, time-efficient, and ultrasensitive detection of disease markers. The PC-SM biochip can enhance the signal-to-noise ratio of single molecules by nearly 3-fold compared with unamplified samples, through coupling of the single-molecule photon energy with the optical band gap of the photonic crystal. We used the PC-SM biochip to detect the low-abundance leukemia inhibitory factor in the blood of pancreatic cancer patients and healthy people and achieved a detection limit of 2.0 pg/L and an AUC of 0.9067. The method exhibits exceptional sensitivity and specificity, showing great application potential in various clinical settings.

Machine Learning-Assisted High-Throughput Identification and Quantification of Protein Biomarkers with Printed Heterochains

Advanced in vitro diagnosis technologies are highly desirable in early detection, prognosis, and progression monitoring of diseases. Here, we engineer a multiplex protein biosensing strategy based on the tunable liquid confinement self-assembly of multi-material heterochains, which show improved sensitivity, throughput, and accuracy compared to standard ELISA kits. By controlling the material combination and the number of ligand nanoparticles (NPs), we observe robust near-field enhancement as well as both strong electromagnetic resonance in polymer-semiconductor heterochains. In particular, their optical signals show a linear response to the coordination number of the semiconductor NPs in a wide range. Accordingly, a visible nanophotonic biosensor is developed by functionalizing antibodies on central polymer chains that can identify target proteins attached to semiconductor NPs. This allows for the specific detection of multiple protein biomarkers from healthy people and pancreatic cancer patients in one step with an ultralow detection limit (1 pg/mL). Furthermore, rapid and high-throughput quantification of protein expression levels in diverse clinical samples such as buffer, urine, and serum is achieved by combining a neural network algorithm, with an average accuracy of 97.3%. This work demonstrates that the heterochain-based biosensor is an exemplary candidate for constructing next-generation diagnostic tools and suitable for many clinical settings.

Synergetic surface enhancement of quantum dots-based electrochemiluminescence with photonic crystal light scattering and metal surface plasmon resonance for sensitive bioanalysis

Noble metal nanostructures and photonic crystals (PhCs) have been widely investigated as substrates for constructing surface enhanced electrochemiluminescence (SE-ECL) biosensors. However, their applications are hindered by the limited enhancement intensity of surface plasmon resonance (SPR) and an incomplete mechanism for the photonic enhancement effect. Hence, developing a novel SE-ECL strategy with better signal enhanced capability and enriching our understanding of the intrinsic mechanisms for efficient bioanalysis is extremely urgent. Here, a synergistic SE-ECL strategy was developed for the sensitive determination of prostate specific antigen (PSA) protein. The randomly arranged polystyrene (r-PS) spheres and PS PhC arrays were applied to enhance the ECL emission of cadmium sulfide quantum dots (CdS QDs) and the results suggested that the PhC arrays displayed superior intensity (0.22) than the r-PS interface (0.10). Au nanoparticles (NPs) were introduced onto the two kinds of surfaces and further boosted the ECL intensity. According to the ECL measurements, Au NPs modified at the r-PS surface exhibited only a slight increase (0.13), while the PhC arrays showed approximately 5-fold enhancement (0.92), benefiting from the synergistic enhancement. The finite-difference time-domain (FDTD) simulation indicated that the ECL enhancement was ascribed to the coupled electromagnetic (EM) field at the surfaces of PS PhCs and Au NPs. The SE-ECL could achieve a detection range from 1 pg/mL to 1 μg/mL with a detection limit of 0.41 pg/mL (S/N = 3). This study provides the first combination of PhC arrays and metal surface plasmon nanostructure for the synergetic enhancement of SE-ECL systems. It opens a new avenue for the rational design of advanced ECL biosensors and shows great perspective for clinical diagnosis.

Printable Light-Emitting Metasurfaces with Enhanced Directional Photoluminescence

Nanoimprint lithography is gaining popularity as a cost-efficient way to reproduce nanostructures in large quantities. Recent advances in nanoimprinting lithography using high-index nanoparticles have demonstrated replication of photonic devices, but it is difficult to confer special properties on nanostructures beyond general metasurfaces. Here, we introduce a novel method for fabricating light-emitting metasurfaces using nanoimprinting lithography. By utilizing quantum dots embedded in resin, we successfully imprint dielectric metasurfaces that function simultaneously as both emitters and resonators. This approach to incorporating quantum dots into metasurfaces demonstrates an improvement in photoluminescence characteristics compared to the situation where quantum dots and metasurfaces are independently incorporated. Design of the metasurface is specifically tailored to support photonic modes within the emission band of quantum dots with a large enhancement of photoluminescence. This study indicates that nanoimprinting lithography has the capability to construct nanostructures using functionalized nanoparticles and could be used in various fields of nanophotonic applications.

Spatial photoluminescence and lifetime mappings of quasi-2D perovskites coupled with a dielectric metasurface

Light-matter interaction between quantum emitters and optical cavities plays a vital role in fundamental quantum photonics and the development of optoelectronics. Resonant metasurfaces are proven to be an efficient platform for tailoring the spontaneous emission (SE) of the emitters. In this work, we study the interplay between quasi-2D perovskites and dielectric TiO2 metasurfaces. The metasurface, functioning as an open cavity, enhances electric fields near its plane, thereby influencing the emissions of the perovskite. This is verified through angle-resolved photoluminescence (PL) studies. We also conducted reflectivity measurements and numerical simulations to validate the coupling between the quasi-2D perovskites and photonic modes. Notably, our work introduces a spatial mapping approach to study Purcell enhancement. Using fluorescence lifetime imaging microscopy (FLIM), we directly link the PL and lifetimes of the quasi-2D perovskites in spatial distribution when positioned on the metasurface. This correlation provides unprecedented insights into emitter distribution and emitter-resonator interactions. The methodology opens a new (to the best of our knowledge) approach for studies in quantum optics, optoelectronics, and medical imaging by enabling spatial mapping of both PL intensity and lifetime, differentiating between uncoupled quantum emitters and those coupled with different types of resonators.

Stretchable Photonic Crystal-Assisted Glycoprotein Identification for Ovarian Cancer Diagnosis

Photonic crystals with specific wavelengths can realize surface-enhanced excitation and emission intensities of fluorophores and enhance the fluorescence signals of fluorescent molecules. Herein, stretchable photonic crystals with good mechanochromic properties provide continuously adjustable forbidden wavelengths by stretching to change the lattice spacing, with reflectance peaks blue-shifted up to 110 nm to match indicators of different wavelengths and produce differentiated optical enhancement effects. Glycoproteins are significantly identified as clinical markers. However, the wide participation of glycoproteins in various life processes poses enormous complexity and critical challenges for rapid, facile, high-throughput, and accurate clinical analysis or health assessment. In this work, we proposed a stretchable photonic crystal-assisted glycoprotein identification approach for early ovarian cancer diagnosis. Stretchable photonic crystals can provide rich optical information to efficiently identify glycoproteins in complex matrices. A double-indicator fluorescence sensor was designed to respond to the protein trunk and oligosaccharide segment of glycoproteins separately for improved recognition accuracy. Seven typical glycoproteins could be discriminated from proteins, saccharides, or mixture interferents. Clinical ovarian cancer samples for early, intermediate, and advanced ovarian cancer and healthy subjects were verified with 100% accuracy. This strategy of stretchable photonic crystal-assisted glycoprotein identification provides an effective method for accurate, rapid ovarian cancer diagnosis and timely clinical treatment.

Enhanced Photocurrent and Electrically Pumped Quantum Dot Emission from Single Plasmonic Nanoantennas

Integrating cavity-enhanced colloidal quantum dots (QDs) into photonic chip devices would be transformative for advancing room-temperature optoelectronic and quantum photonic technologies. However, issues with efficiency, stability, and cost remain formidable challenges to reach the single antenna limit. Here, we present a bottom-up approach that delivers single QD-plasmonic nanoantennas with electrical addressability. These QD nanojunctions exhibit robust photoresponse characteristics, with plasmonically enhanced photocurrent spectra matching the QD solution absorption. We demonstrate electroluminescence from individual plasmonic nanoantennas, extending the device lifetime beyond 40 min by utilizing a 3 nm electron-blocking polymer layer. In addition, we reveal a giant voltage-dependent redshift of up to 62 meV due to the quantum-confined Stark effect and determine the exciton polarizability of the CdSe QD monolayer to be 4 × 10-5 meV/(kV/cm)2. These developments provide a foundation for accessing scalable quantum light sources and high-speed, tunable optoelectronic systems operating under ambient conditions.

Fluorescent Selectivity-Enhanced FRET Based on 3D Photonic Crystals for Multianalyte Sensing

Fluorescence resonance energy transfer (FRET) finds widespread utility in biochemical sensing, single-molecule experiments, cell physiology, and various other domains due to its inherent simplicity and high sensitivity. Nevertheless, the efficiency of energy transfer between the FRET donor and acceptor is significantly contingent on the local photonic environment, a factor that limits its application in complex systems or multianalyte detections. Here, a fluorescent selectivity-enhanced acridine orange (AO)-aflatoxins (AFs) FRET system based on a range of 3D topological photonic crystals (PCs) was developed with the aim of enhancing the selectivity and discrimination capabilities of FRET. By exploring the angle-dependent characteristics of the photonic stopband, the stopband distribution across different 3D topological PCs pixels was investigated. This approach led to selective fluorescence enhancement in PCs that matched the stopbands, enabling the successful discrimination of six distinct aflatoxins and facilitating complex multianalysis of moldy food samples. In particular, the stopband, which was strategically positioned within the blue-purple structural color range, exhibited a strong alignment with the fluorescence peaks of both the FRET donor and acceptor. This alignment allowed the 3D three-pointed star PCs to be effectively employed for the identification of mixed samples containing six distinct aflatoxins as well as the detection of real aflatoxin samples present in moldy potatoes, bread, oats, and peanuts. Impressively, this approach achieved a remarkable accuracy rate of 100%. This innovative strategy not only presents a novel avenue for developing a multitarget discrimination analysis system but also offers a convenient pretreatment method for the quantitative detection of various aflatoxins.

Ultrasensitive Upconversion Nanoparticle Immunoassay for Human Serum Cardiac Troponin I Detection Achieved with Resonant Waveguide Grating

Selective detection of biomarkers at low concentrations in blood is crucial for the clinical diagnosis of many diseases but remains challenging. In this work, we aimed to develop an ultrasensitive immunoassay that can detect biomarkers in serum with an attomolar limit of detection (LOD). We proposed a sandwich-type heterogeneous immunosensor in a 3 × 3 well array format by integrating a resonant waveguide grating (RWG) substrate with upconversion nanoparticles (UCNPs). UCNPs were used to label a target biomarker captured by capture antibody molecules immobilized on the surface of the RWG substrate, and the RWG substrate was used to enhance the upconversion luminescence (UCL) of UCNPs through excitation resonance. The LOD of the immunosensor was greatly reduced due to the increased UCL of UCNPs and the reduction of nonspecific adsorption of detection antibody-conjugated UCNPs on the RWG substrate surface by coating the RWG substrate surface with a carboxymethyl dextran layer. The immunosensor exhibited an extremely low LOD [0.24 fg/mL (9.1 aM)] and wide detection range (1 fg/mL to 100 pg/mL) in the detection of cardiac troponin I (cTnI). The cTnI concentrations in human serum samples collected at different times during cyclophosphamide, epirubicin, and 5-fluorouracil (CEF) chemotherapy in a breast cancer patient were measured by an immunosensor, and the results showed that the CEF chemotherapy did cause cardiotoxicity in the patient. Having a higher number of wells in such an array-based biosensor, the sensor can be developed as a high-throughput diagnostic tool for clinically important biomarkers.

A Universal Strategy for Enhancing the Circulating miRNAs' Detection Performance of Rolling Circle Amplification by Using a Dual-Terminal Stem-Loop Padlock

Rolling circle amplification (RCA) is one of the most promising nucleic acid detection technologies and has been widely used in the molecular diagnosis of disease. Padlock probes are often used to form circular templates, which are the core of RCA. However, RCA often suffers from insufficient specificity and sensitivity. Here we report a reconstruction strategy for conventional padlock probes to promote their overall performance in nucleic acid detection while maintaining probe functions uncompromised. When two rationally designed stem-loops were strategically placed at the two terminals of linear padlock probes, the specificity of target recognition was enhanced and the negative signal was significantly delayed. Our design achieved the best single-base discrimination compared with other structures and over a 1000-fold higher sensitivity than that of the conventional padlock probe, validating the effectiveness of this reconstruction. In addition, the underlying mechanisms of our design were elucidated through molecular dynamics simulations, and the versatility was validated with longer and shorter padlocks targeting the same target, as well as five additional targets (four miRNAs and dengue virus - 2 RNA mimic (DENV-2)). Finally, clinical applicability in multiplex detection was demonstrated by testing real plasma samples. Our exploration of the structures of nucleic acids provided another perspective for developing high-performance detection systems, improving the efficacy of practical detection strategies, and advancing clinical diagnostic research.

Dual-Resonance Nanostructures for Color Downconversion of Colloidal Quantum Emitters

We present a dual-resonance nanostructure made of a titanium dioxide (TiO2) subwavelength grating to enhance the color downconversion efficiency of CdxZn1-xSeyS1-y colloidal quantum dots (QDs) emitting at ∼530 nm when excited with a blue light at ∼460 nm. A large mode volume can be created within the QD layer by the hybridization of the grating resonances and waveguide modes, resulting in large absorption and emission enhancements. Particularly, we achieved polarized light emission with a maximum photoluminescence enhancement of ∼140 times at a specific angular direction and a total enhancement of ∼34 times within a 0.55 numerical aperture (NA) of the collecting objective. The enhancement encompasses absorption, Purcell and outcoupling enhancements. We achieved a total absorption of 35% for green QDs with a remarkably thin color conversion layer of ∼400 nm. This work provides a guideline for designing large-volume cavities for absorption/fluorescence enhancement in microLED display, detector, or photovoltaic applications.

Photonic-Plasmonic Coupling Enhanced Fluorescence Enabling Digital-Resolution Ultrasensitive Protein Detection

Assays utilizing fluorophores are common throughout life science research and diagnostics, although detection limits are generally limited by weak emission intensity, thus requiring many labeled target molecules to combine their output to achieve higher signal-to-noise. We describe how the synergistic coupling of plasmonic and photonic modes can significantly boost the emission from fluorophores. By optimally matching the resonant modes of a plasmonic fluor (PF) nanoparticle and a photonic crystal (PC) with the absorption and emission spectrum of the fluorescent dye, a 52-fold improvement in signal intensity is observed, enabling individual PFs to be observed and digitally counted, where one PF tag represents one detected target molecule. The amplification can be attributed to the strong near-field enhancement due to the cavity-induced activation of the PF, PC band structure-mediated improvement in collection efficiency, and increased rate of spontaneous emission. The applicability of the method by dose-response characterization of a sandwich immunoassay for human interleukin-6, a biomarker used to assist diagnosis of cancer, inflammation, sepsis, and autoimmune disease is demonstrated. A limit of detection of 10 fg mL-1 and 100 fg mL-1 in buffer and human plasma respectively, is achieved, representing a capability nearly three orders of magnitude lower than standard immunoassays.

Toward Chiral Lasing from All-Solution-Processed Flexible Perovskite-Nanocrystal-Liquid-Crystal Membranes

Circularly polarized (CP) coherent light sources are of great potential for various advanced optical applications spanning displays/imaging to data processing/encryption and quantum communication. Here, the first demonstration of CP amplified spontaneous emission (ASE)/lasing from a free-standing and flexible membrane device is reported. The membrane device consists of perovskite nanocrystals (PNCs) and cholesteric liquid crystals (CLCs) layers sandwiched within a Fabry-Pérot (F-P) cavity architecture. The chiral liquid crystal cavity enables the generation of CP light from the device. The device is completely solution-processable and displays CP ASE with record dissymmetry factor (glum ) as high as 1.4, which is 3 orders of magnitude higher as compared with glum of CP luminescence of chiral ligand-capped colloidal PNCs. The device exhibits ultraflexibility as the ASE intensity remains unchanged after repeated 100 bending cycles and it is stable for more than 3 months with 80% of its original intensity. Furthermore, the ultraflexibility enables the generation of ASE from various objects of different geometric surfaces covered with the flexible perovskite membrane device. This work not only demonstrates the first CP ASE from a PNCs membrane with extremely high glum but also opens the door toward the fabrication of ultraflexible, extremely stable, and all solution-processable perovskite chiral laser devices.

Pseudomorphic Synthesis of Monodisperse Afterglow Carbon Dot-Doped SiO2 Microparticles for Photonic Crystals

Synthesizing monodisperse afterglow microparticles (MPs) is crucial for creating photonic crystal (PC) platforms with multiple optical states for optoelectronics. However, achieving high uniformity in both size and morphology is challenging for inorganic afterglow MPs using conventional methods. In this contribution, a novel approach for the synthesis of carbon dot (CD)-doped SiO2 MPs with tunable afterglow properties and size distributions is reported. These mechanism studies suggest that the pseudomorphic transformation of SiO2 MPs enables CD doping, providing a hydrogen bond-enriched environment for triplet state stabilization, which generates green afterglow while retaining the uniformity in size and morphology of the parent SiO2 MPs. Furthermore, the utility of CD-doped SiO2 MPs in the fabrication of rationally designed PC patterns is shown using a combined consecutive dip-coating and laser-assisted etching strategy. The pattern displays multiple optical responses under different lighting conditions, including angle-dependent structural colors and blue luminescence under daylight and upon 365-nm irradiation, respectively, as well as time-dependent green afterglow after ceasing UV excitation. The findings pave the way for further controlling the dynamics of spontaneous emissions by PCs to enable complicated optical states for advanced photonics.

Application of colloidal photonic crystals in study of organoids

As alternative disease models, other than 2D cell lines and patient-derived xenografts, organoids have preferable in vivo physiological relevance. However, both endogenous and exogenous limitations impede the development and clinical translation of these organoids. Fortunately, colloidal photonic crystals (PCs), which benefit from favorable biocompatibility, brilliant optical manipulation, and facile chemical decoration, have been applied to the engineering of organoids and have achieved the desirable recapitulation of the ECM niche, well-defined geometrical onsets for initial culture, in situ multiphysiological parameter monitoring, single-cell biomechanical sensing, and high-throughput drug screening with versatile functional readouts. Herein, we review the latest progress in engineering organoids fabricated from colloidal PCs and provide inputs for future research.

Adaptive time modulation technique for multiplexed on-chip particle detection across scales

Integrated optofluidic biosensors have demonstrated ultrasensitivity down to single particle detection and attomolar target concentrations. However, a wide dynamic range is highly desirable in practice and can usually only be achieved by using multiple detection modalities or sacrificing linearity. Here, we demonstrate an analysis technique that uses temporal excitation at two different time scales to simultaneously enable digital and analog detection of fluorescent targets. We demonstrated the seamless detection of nanobeads across eight orders of magnitude from attomolar to nanomolar concentration. Furthermore, a combination of spectrally varying modulation frequencies and a closed-loop feedback system that provides rapid adjustment of excitation laser powers enables multiplex analysis in the presence of vastly different concentrations. We demonstrated this ability to detect across scales via an analysis of a mixture of fluorescent nanobeads at femtomolar and picomolar concentrations. This technique advances the performance and versatility of integrated biosensors, especially toward point-of-use applications.

Photonic crystal enhanced immunofluorescence biosensor integrated with a lateral flow microchip: Toward rapid tear-based diabetic retinopathy screening

Diabetic retinopathy (DR) has accounted for major loss of vision in chronic diabetes. Although clinical statistics have shown that early screening can procrastinate or improve the deterioration of the disease, the screening rate remains low worldwide because of the great inconvenience of conventional ophthalmoscopic examination. Instead, tear fluid that contains rich proteins caused by direct contact with eyeballs is an ideal substitute to monitor vision health. Herein, an immunofluorescence biosensor enhanced by a photonic crystal (PhC) is presented to handle the trace proteins suspended in the tear fluid. The PhC was constructed by self-assembled nanoparticles with a thin layer of gold coated on top of it. Then, the PC substrate was conjugated with antibodies and placed in a microchannel. When the capillary-driven tear sample flew over the PC substrate, the immunoassay enabled the formation of a sandwich antibody-antigen-antibody configuration for PhC-enhanced immunofluorescence. The use of PhC resulted in a concentration enhancement of more than tenfold compared to non-PhC, while achieving an equivalent signal intensity. The limit of detection for the target biomarker, lipocalin-1 (LCN-1), reached nearly 3 μg/ml, and the turnaround time of each detection was 15 min. Finally, a preclinical evaluation was conducted using ten tear samples. A clear trend was observed, showing that the concentrations of LCN-1 were at least twofold higher in individuals with chronic diabetes or DR than in healthy individuals. This trend was consistent with their medical conditions. The results provided a direct proof-of-concept for the proposed PhC biosensor in rapid tear-based DR screening.

Digital and Absolute Assays for Low Abundance Molecular Biomarkers

There has been a recent surge of advances in biomolecular assays based on the measurement of discrete molecular targets as opposed to signals averaged across molecular ensembles. Many of these "digital" assay designs derive from now-mature technologies involving single-molecule imaging and microfluidics and provide an assortment of new modalities to quantify nucleic acids and proteins in biospecimens such as blood and tissue homogenates. A primary new benefit is the robust detection of trace analytes at attomolar to femtomolar concentrations for which many ensemble assays cannot distinguish signals above noise levels. In addition, multiple biomolecules can be differentiated within a mixture using optical barcodes, with much faster and simpler readouts compared with sequencing methods. In ideal digital assays, signals should, in theory, further represent absolute molecular counts, rather than relative levels, eliminating the need for calibration standards that are the mainstay of typical assays. Several digital assay platforms have now been commercialized but challenges hinder the adoption and diversification of these new formats, as there are broad needs to balance sensitivity and dynamic range of detection, increase analyte multiplexing, improve sample throughput, and reduce cost. Our lab and others have developed technologies to address these challenges by redesigning molecular probes and labels, improving molecular transport within detection focal volumes, and applying solution-based readout methods in flow.This Account describes the principles, formats, and design constraints of digital biomolecular assays that apply optical labels toward the goal of simple and routine target counting that may ultimately approach absolute readout standards. The primary challenges can be understood from fundamental concepts in thermodynamics and kinetics of association reactions, mass transport, and discrete statistics. Major advances include (1) new inorganic nanocrystal probes for more robust counting compared with dyes, (2) diverse molecular amplification tools that endow attachment of numerous labels to single targets, (3) specialized surfaces with patterned features for electromagnetic coupling to labels for signal amplification, (4) surface capture enhancement methods to concentrate targets through disruption of diffusion depletion zones, and (5) flow counting in which analytes are rapidly counted in solution without pull-down to a surface. Further progress and integration of these tools for biomolecular counting could improve the precision of laboratory measurements in life sciences research and benefit clinical diagnostic assays for low abundance biomarkers in limiting biospecimen volumes that are out of reach of traditional ensemble-level bioassays.

Designer DNA NanoGripper

DNA has shown great biocompatibility, programmable mechanical properties, and structural addressability at the nanometer scale, making it a versatile material for building high precision nanorobotics for biomedical applications. Herein, we present design principle, synthesis, and characterization of a DNA nanorobotic hand, called the "NanoGripper", that contains a palm and four bendable fingers as inspired by human hands, bird claws, and bacteriophages evolved in nature. Each NanoGripper finger has three phalanges connected by two flexible and rotatable joints that are bendable in response to binding to other entities. Functions of the NanoGripper have been enabled and driven by the interactions between moieties attached to the fingers and their binding partners. We showcase that the NanoGripper can be engineered to interact with and capture various objects with different dimensions, including gold nanoparticles, gold NanoUrchins, and SARS-CoV-2 virions. When carrying multiple DNA aptamer nanoswitches programmed to generate fluorescent signal enhanced on a photonic crystal platform, the NanoGripper functions as a sensitive viral biosensor that detects intact SARS-CoV-2 virions in human saliva with a limit of detection of ~ 100 copies/mL, providing RT-PCR equivalent sensitivity. Additionally, we use confocal microscopy to visualize how the NanoGripper-aptamer complex can effectively block viral entry into the host cells, indicating the viral inhibition. In summary, we report the design, synthesis, and characterization of a complex nanomachine that can be readily tailored for specific applications. The study highlights a path toward novel, feasible, and efficient solutions for the diagnosis and therapy of other diseases such as HIV and influenza.

First-passage time analysis of diffusion-controlled reactions in single-molecule detection

Single-molecule detection (SMD) aims to achieve the ultimate limit-of-detection (LOD) in biosensing. This method detects a countable number of targeted analyte molecules in solution, where the dynamics of molecule diffusion, capturing, identification and delivery greatly impact the SMD's efficiency and accuracy. In this study, we adopt the first-passage time method to investigate the diffusion-controlled reaction process in SMD. We analyze the influence of detection conditions on incubation time and the expected coefficient of variation (CV) under three SMD molecule capturing strategies, including solid-phase capturing (one-dimensional solid-liquid interface fixation), liquid-phase magnetic bead (MB) capturing, and liquid-phase direct fluorescence pair labeling. We find that inside a finite-sized reaction chamber, a finite average reaction time exists in all three capturing strategies, while the liquid-phase strategies are in general more efficient than the solid-phase approaches. CV can be estimated by averaging first-passage time solely in all three strategies, and the CV reduction is achievable given an extended reaction time. To further enable zeptomolar detection, extra treatments, such as adopting liquid-phase fluorescence pairs with high diffusion rates to label the molecule, or designing specific sensing devices with large effective sensing areas would be required. This framework provides solid theoretical support to guide the design of SMD sensing strategies and sensor structures to achieve desired measurement time and CV.

Photonic Crystal Enhanced Fluorescence: A Review on Design Strategies and Applications

Nanoscale fluorescence emitters are efficient for measuring biomolecular interactions, but their utility for applications requiring single-unit observations is constrained by the need for large numerical aperture objectives, fluorescence intermittency, and poor photon collection efficiency resulting from omnidirectional emission. Photonic crystal (PC) structures hold promise to address the aforementioned challenges in fluorescence enhancement. In this review, we provide a broad overview of PCs by explaining their structures, design strategies, fabrication techniques, and sensing principles. Furthermore, we discuss recent applications of PC-enhanced fluorescence-based biosensors incorporated with emerging technologies, including nucleic acids sensing, protein detection, and steroid monitoring. Finally, we discuss current challenges associated with PC-enhanced fluorescence and provide an outlook for fluorescence enhancement with photonic-plasmonics coupling and their promise for point-of-care biosensing as well monitoring analytes of biological and environmental relevance. The review presents the transdisciplinary applications of PCs in the broad arena of fluorescence spectroscopy with broad applications in photo-plasmonics, life science research, materials chemistry, cancer diagnostics, and internet of things.

Optical Biosensors and Their Applications for the Detection of Water Pollutants

The correct detection and quantification of pollutants in water is key to regulating their presence in the environment. Biosensors offer several advantages, such as minimal sample preparation, short measurement times, high specificity and sensibility and low detection limits. The purpose of this review is to explore the different types of optical biosensors, focusing on their biological elements and their principle of operation, as well as recent applications in the detection of pollutants in water. According to our literature review, 33% of the publications used fluorescence-based biosensors, followed by surface plasmon resonance (SPR) with 28%. So far, SPR biosensors have achieved the best results in terms of detection limits. Although less common (22%), interferometers and resonators (4%) are also highly promising due to the low detection limits that can be reached using these techniques. In terms of biological recognition elements, 43% of the published works focused on antibodies due to their high affinity and stability, although they could be replaced with molecularly imprinted polymers. This review offers a unique compilation of the most recent work in the specific area of optical biosensing for water monitoring, focusing on both the biological element and the transducer used, as well as the type of target contaminant. Recent technological advances are discussed.

Digital detection of proteins

This paper reviews methods for detecting proteins based on molecular digitization, i.e., the isolation and detection of single protein molecules or singulated ensembles of protein molecules. The single molecule resolution of these methods has resulted in significant improvements in the sensitivity of immunoassays beyond what was possible using traditional "analog" methods: the sensitivity of some digital immunoassays approach those of methods for measuring nucleic acids, such as the polymerase chain reaction (PCR). The greater sensitivity of digital protein detection has resulted in immuno-diagnostics with high potential societal impact, e.g., the early diagnosis and therapeutic intervention of Alzheimer's Disease. In this review, we will first provide the motivation for developing digital protein detection methods given the limitations in the sensitivity of analog methods. We will describe the paradigm shift catalyzed by single molecule detection, and will describe in detail one digital approach - which we call digital bead assays (DBA) - based on the capture and labeling of proteins on beads, identifying "on" and "off" beads, and quantification using Poisson statistics. DBA based on the single molecule array (Simoa) technology have sensitivities down to attomolar concentrations, equating to ∼10 proteins in a 200 μL sample. We will describe the concept behind DBA, the different single molecule labels used, the ways of analyzing beads (imaging of arrays and flow), the binding reagents and substrates used, and integration of these technologies into fully automated and miniaturized systems. We provide an overview of emerging approaches to digital protein detection, including those based on digital detection of nucleic acids labels, single nanoparticle detection, measurements using nanopores, and methods that exploit the kinetics of single molecule binding. We outline the initial impact of digital protein detection on clinical measurements, highlighting the importance of customized assay development and translational clinical research. We highlight the use of DBA in the measurement of neurological protein biomarkers in blood, and how these higher sensitivity methods are changing the diagnosis and treatment of neurological diseases. We conclude by summarizing the status of digital protein detection and suggest how the lab-on-a-chip community might drive future innovations in this field.

Biosensing Technologies: A Focus Review on Recent Advancements in Surface Plasmon Coupled Emission

In the past decade, novel nano-engineering protocols have been actively synergized with fluorescence spectroscopic techniques to yield higher intensity from radiating dipoles, through the process termed plasmon-enhanced fluorescence (PEF). Consequently, the limit of detection of analytes of interest has been dramatically improvised on account of higher sensitivity rendered by augmented fluorescence signals. Recently, metallic thin films sustaining surface plasmon polaritons (SPPs) have been creatively hybridized with such PEF platforms to realize a substantial upsurge in the global collection efficiency in a judicious technology termed surface plasmon-coupled emission (SPCE). While the process parameters and conditions to realize optimum coupling efficiency between the radiating dipoles and the plasmon polaritons in SPCE framework have been extensively discussed, the utility of disruptive nano-engineering over the SPCE platform and analogous interfaces such as 'ferroplasmon-on-mirror (FPoM)' as well as an alternative technology termed 'photonic crystal-coupled emission (PCCE)' have been seldom reviewed. In light of these observations, in this focus review, the myriad nano-engineering protocols developed over the SPCE, FPoM and PCCE platform are succinctly captured, presenting an emphasis on the recently developed cryosoret nano-assembly technology for photo-plasmonic hotspot generation (first to fourth). These technologies and associated sensing platforms are expected to ameliorate the current biosensing modalities with better understanding of the biophysicochemical processes and related outcomes at advanced micro-nano-interfaces. This review is hence envisaged to present a broad overview of the latest developments in SPCE substrate design and development for interdisciplinary applications that are of relevance in environmental as well as biological heath monitoring.

Refractive Index Dependence of Fluorescence Enhancement in a Nanostructured Plasmonic Grating

Plasmonic gratings are attracting huge interest in the context of realizing sensors based on surface-enhanced fluorescence. The grating features control the plasmonic modes and consequently have a strong effect on the fluorescence response. Within this framework, we focused on the use of a buffer solution flowing across the grating active surface to mimic a real measurement. The refractive index of the surrounding medium is therefore altered, with a consequent modification of the resonance conditions. The result is a shift in the spectral features of the fluorescence emission accompanied by a reshaping of the fluorescence emission in terms of spectral weight and direction.

Cactus-Inspired Photonic Crystal Chip for Attomolar Fluorescence Multi-analysis

Automation and efficiency requirements of environmental monitoring are the pursuit of spontaneous sampling and ultrasensitivity for current sensory systems or detection apparatuses. In this work, inspired by cactus hierarchical structures, we develop a cactus-inspired photonic crystal chip to integrate spontaneous droplet sampling and fluorescence enhancement for sensitive multi-analyte detection. A conical hydrophilic pattern on hydrophobic surfaces can give rise to unidirectional Laplace pressure, which drives droplet transport to the assigned photonic crystal site. The nanostructure of photonic crystals has bigger capillarity to drive the droplet wetting uniformly into the photonic crystal matrix while performing prominent fluorescence enhancement by their photonic bandgap. A low to attomolar (2.24 × 10-19 M) fluorescence limit of detection (LOD) sensitivity can be achieved by the synergy of spontaneous droplet sampling and fluorescence enhancement. Focused on eutrophic water problems and algae pollution monitoring, a femtomolar (1.83 × 10-15 M) LOD and identification of various microcystins in urban environmental water can be achieved. The suitable integration of the unidirectional droplet transport by Laplace pressure and fluorescence enhancement by photonic crystals can achieve the spontaneous sampling and signal enhancement for ultratrace detections and sample survey of environmental monitoring and disease diagnosis.

Deep ultraviolet spontaneous emission enhanced by layer dependent black phosphorus plasmonics

Although graphene has been the primary material of interest recently for spontaneous emission engineering through the Purcell effect, it features isotropic and thickness-independent optical properties. In contrast, the optical properties of black Phosphorus (BP) are in-plane anisotropic; which supports plasmonic modes and are thickness-dependent, offering an additional degree of freedom for control. Here we investigate how the anisotropy and thickness of BP affect spontaneous emission from a Hydrogenic emitter. We find that the spontaneous emission enhancement rate i.e. Purcell factor (PF) depends on emitter orientation, and PF at a particular frequency and distance can be controlled by BP thickness. At lower frequencies, PF increases with increasing thickness due to infrared (IR) plasmons, which then enhances visible and UV far-field spectra, even at energies greater than 10 eV. By leveraging the thickness and distance-dependent PF, deep UV emission can be switched between 103 nm or 122 nm wavelength from a Hydrogenic emitter. Additionally, we find that doping can significantly tune the PF near BP and this alteration depends on the thickness of the BP. Our work shows that BP is a promising platform for studying strong plasmon-induced light-matter interactions tunable by varying doping levels, emitter orientation, and thickness.

Plasmonic Modes and Fluorescence Enhancement Coupling Mechanism: A Case with a Nanostructured Grating

The recent development and technological improvement in dealing with plasmonic metasurfaces has triggered a series of interesting applications related to sensing challenges. Fluorescence has been one of the most studied tools within such a context. With this in mind, we used some well characterized structures supporting plasmonic resonances to study their influence on the emission efficiency of a fluorophore. An extended optical analysis and a complementary investigation through finite-difference time-domain (FDTD) simulations have been combined to understand the coupling mechanism between the excitation of plasmonic modes and the fluorescence absorption and emission processes. The results provide evidence of the spectral shape dependence of fluorescence on the plasmonic field distribution together with a further relationship connected with the enhancement of its signal. It has made evident that the spectral region characterized by the largest relative enhancement closely corresponds to the strongest signatures of the plasmonic modes, as described by both the optical measurements and the FDTD findings.

Single-Particle Optical Imaging for Ultrasensitive Bioanalysis

The quantitative detection of critical biomolecules and in particular low-abundance biomarkers in biofluids is crucial for early-stage diagnosis and management but remains a challenge largely owing to the insufficient sensitivity of existing ensemble-sensing methods. The single-particle imaging technique has emerged as an important tool to analyze ultralow-abundance biomolecules by engineering and exploiting the distinct physical and chemical property of individual luminescent particles. In this review, we focus and survey the latest advances in single-particle optical imaging (OSPI) for ultrasensitive bioanalysis pertaining to basic biological studies and clinical applications. We first introduce state-of-the-art OSPI techniques, including fluorescence, surface-enhanced Raman scattering, electrochemiluminescence, and dark-field scattering, with emphasis on the contributions of various metal and nonmetal nano-labels to the improvement of the signal-to-noise ratio. During the discussion of individual techniques, we also highlight their applications in spatial-temporal measurement of key biomarkers such as proteins, nucleic acids and extracellular vesicles with single-entity sensitivity. To that end, we discuss the current challenges and prospective trends of single-particle optical-imaging-based bioanalysis.

Solvent-Responsive Invisible Photonic Patterns with High Contrast for Fluorescence Emission Regulation and Anti-Counterfeiting

Invisible photonic patterns (IPPs) are photonic materials that can display hidden patterns under external stimulation and are attractive in anti-counterfeiting devices and information storage. In this work, we report a solvent-responsive invisible photonic pattern (SRIPP) with high contrast by polymerizing two monomers of acrylamide (AAm) and poly(ethylene glycol) methacrylate (PEGMA) with different solubility parameters in different regions of poly(hydroxyethyl methacrylate) photonic gels. The two regions with different solvent responsiveness can shrink and swell in the same environment, thus causing the colors of different regions of photonic gel to shift in opposite directions from the initial state. As a result, the contrast of photonic patterns is significantly improved, increasing naked-eye visualization. In addition, by introducing fluorescent substances into the photonic gel and adjusting the photonic band gap (PBG) of photonic gels, we realize the regulation of fluorescence emission and display of fluorescence patterns by utilizing different PBGs on the SRIPP. Dynamic solvent responsiveness patterns and fluorescence patterns are integrated into a photonic gel, showing great potential in information storage and multiple-mode anti-counterfeiting applications.

Sericin-Based Bio-Inspired Nano-Engineering of Heterometallic AgAu Nanocubes for Attomolar Mefenamic Acid Sensing in the Mobile Phone-Based Surface Plasmon-Coupled Interface

Engineering photo-plasmonic platforms with heterometallic nanohybrids are of paramount significance for realizing augmented sensitivity in fluorescence-based analytical detection. Although myriad nanomaterials with versatile functionalities have been explored in this regard in the surface plasmon-coupled emission (SPCE) interface, light harvesting using nano-antennas synthesized via sustainable bio-inspired routes still remains a high priority in current research. Our study provides a rational design for in situ fabrication of nanoparticles of silver, gold, and their plasmonic hybrids using biocompatible, non-hazardous sericin protein (obtained Bombyx mori) as the reducing and capping agent. The one-pot, user-eco-friendly technology demonstrated here utilizes UV irradiation to promote the photo-induced electron transfer mechanism, thereby yielding nanomaterials of tunable optoelectronic functionalities. The resulting homometallic and heterometallic nanohybrids with robust localized surface plasmon resonances (LSPR) showed strong light-confining attributes when interfaced with the propagating surface plasmon polaritons (SPPs) of the SPCE platform, thereby yielding tunable, highly directional, polarized, and amplified fluorescence emission. The experimentally obtained emission profiles displayed an excellent correlation with the theoretically obtained dispersion diagrams validating the spectro-plasmonic results. The abundant hotspots from AgAu nanocubes presented in excess of 1300-fold dequenched fluorescence enhancement and were utilized for cost-effective and real-time mobile phone-based sensing of biologically relevant mefenamic acid at an attomolar limit of detection. We believe that this superior biosensing performance accomplished using the frugal bioinspired nano-engineering at hybrid interfaces would open new doors for developing nanofabrication protocols with the quintessential awareness of the principles of green nanotechnology, consequently eliminating hazardous chemicals and solvents in the development of point-of-care diagnostic tools.