Label-Free Probing of Molecule Binding Kinetics Using Single-Particle Interferometric Imaging

Gold nanoparticle-decorated fluorine-doped tin oxide substrate for sensitive label-free OIRD microarray chips

Oblique incidence reflectance difference (OIRD) is an emerging technique enabling real-time and label-free detection of bio-affinity binding events on microarrays. The interfacial architecture of the microarray chip is critical to the performance of OIRD detection. In this work, a sensitive label-free OIRD microarray chip was developed by using gold nanoparticle-decorated fluorine-doped tin oxide (AuNPs-FTO) slides as a chip substrate. This AuNPs-FTO chip demonstrates a higher signal-to-noise ratio and improved sensitivity compared to that built on FTO glass, showing a detection limit of as low as 10 ng mL-1 for the model target, HRP-conjugated streptavidin. On-chip ELISA experiments and optical calculations suggest that the enhanced performance is not only due to the higher probe density enabling a high capture efficiency toward the target, but most importantly, the AuNP layer arouses optical interference to improve the intrinsic sensitivity of OIRD. This work provides an effective strategy for constructing OIRD-based microarray chips with enhanced sensitivity, and may help extend their practical applications in various fields.

Surface Plasmon Resonance Biosensors: A Review of Molecular Imaging with High Spatial Resolution

Surface plasmon resonance (SPR) is a powerful tool for determining molecular interactions quantitatively. SPR imaging (SPRi) further improves the throughput of SPR technology and provides the spatially resolved capability for observing the molecular interaction dynamics in detail. SPRi is becoming more and more popular in biological and chemical sensing and imaging. However, SPRi suffers from low spatial resolution due to the imperfect optical components and delocalized features of propagating surface plasmonic waves along the surface. Diverse kinds of approaches have been developed to improve the spatial resolution of SPRi, which have enormously impelled the development of the methodology and further extended its possible applications. In this minireview, we introduce the mechanisms for building a high-spatial-resolution SPRi system and present its experimental schemes from prism-coupled SPRi and SPR microscopy (SPRM) to surface plasmonic scattering microscopy (SPSM); summarize its exciting applications, including molecular interaction analysis, molecular imaging and profiling, tracking of single entities, and analysis of single cells; and discuss its challenges in recent decade as well as the promising future.

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.

Label-free OIRD microarray chips with a nanostructured sensing interface: enhanced sensitivity and mechanism

Oblique-incidence reflectivity difference (OIRD) is a novel optical technique for protein microarray detection with the characteristics of being real-time, label-free, high-throughput and compatible with arbitrary chip substrates. It is necessary yet challenging to improve the sensitivity of the OIRD microarray and gain a clear understanding of the enhancement mechanism for practical applications. In this study, we report a microarray chip specifically designed for OIRD to improve its sensitivity by using an electrochemically etched nanostructured fluorine-doped tin oxide (FTO) slide as the substrate. Compared with chips printed on a conventional glass slide and pristine FTO, the OIRD sensitivity and signal-to-noise ratio of this microarray are significantly improved, reaching a limit of detection (LOD) as low as 50 ng mL^-1 for the streptavidin target in 10% human serum, which is one order of magnitude lower than that of the glass-based chip. On-chip ELISA and theoretical calculation reveal that the enhanced sensitivity is not only because of its higher capture efficiency towards the target, but also benefits from the optical enhancement enabled by its unique nanostructured sensing interface. This work provides a new universal strategy for designing high performance OIRD-based chips via rational interfacial engineering, thus paving the way to a label-free OIRD immunoassay and real-time analysis of biomolecular interactions.

Imaging the Heterogeneous Localization of a Single Molecule

Single-molecule localization allows determining the underlying biological and biochemical processes and promotes the development of super-resolution imaging techniques. Here, we present an optical technique of tracking the motion of a single nanoparticle linked to a substrate via a biomolecule tether to reveal the localization of single biomolecules and the transient states of single nanoparticle switching between specific binding pairs. The affinities, steric hindrance, and conformational variation of a single-molecule binding pair uncover the dynamic details and intrinsic mechanism of binding processes with high specificity and accuracy (a few nanometers). The application of tracking motions of single soft liposomes on different modified surfaces was further demonstrated, which revealed the characteristic behavior related to surface chemistry. Our results show that the trajectory of nanoscale liposomes loaded with small-drug molecules is linked to the compositional inhomogeneity, which provides a route for thorough comprehension of the fundamental biotechnological process.