Dual-mode refractive index and charge sensing to investigate complex surface chemistry on nanostructures

Localized Surface Plasmon Resonance Sensing and its Interplay with Fluidics

In this Feature Article, we discuss the interplay between fluidics and the localized surface plasmon resonance (LSPR) sensing technique, primarily focusing on its applications in the realm of bio/chemical sensing within fluidic environments. Commencing with a foundational overview of LSPR principles from a sensing perspective, we subsequently showcase the development of a streamlined LSPR chip integrated with microfluidic structures. This integration opens the doors to advanced experiments involving fluid dynamics, greatly expanding the scope of LSPR-based research. Our discussions then turn to the practical implementation of LSPR and microfluidics in real-time biosensing, with a specific emphasis on monitoring DNA polymerase activity. Additionally, we illustrate the direct sensing of biological fluids, exemplified by the analysis of urine, while also shedding light on a unique particle assembly process that occurs on LSPR chips. We not only discuss the significance of LSPR sensing but also explore its potential to investigate a plethora of phenomena at liquid-liquid and solid-liquid interfaces. This is particularly noteworthy, as existing transduction methods and sensors fall short in fully comprehending these interfacial phenomena. Concluding our discussion, we present a futuristic perspective that provides insights into potential opportunities emerging at the intersection of fluidics and LSPR sensing.

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.

Recent Advances in Aptasensor for Cytokine Detection: A Review

Cytokines are proteins secreted by immune cells. They promote cell signal transduction and are involved in cell replication, death, and recovery. Cytokines are immune modulators, but their excessive secretion causes uncontrolled inflammation that attacks normal cells. Considering the properties of cytokines, monitoring the secretion of cytokines in vivo is of great value for medical and biological research. In this review, we offer a report on recent studies for cytokine detection, especially studies on aptasensors using aptamers. Aptamers are single strand nucleic acids that form a stable three-dimensional structure and have been receiving attention due to various characteristics such as simple production methods, low molecular weight, and ease of modification while performing a physiological role similar to antibodies.

Covalent immobilization of gold nanoparticles on a plastic substrate and subsequent immobilization of biomolecules

We propose a novel approach to stably immobilize gold nanoparticles (AuNPs) on a plastic substrate and demonstrate that the modified substrate is also capable of immobilizing biomolecules. To immobilize citrate-capped AuNPs, an acrylic substrate was simply dip-coated in a functional polymer solution to decorate the outermost surface with amino groups. Electrostatic interactions between AuNPs and the amino groups immobilized the AuNPs with a high density. The AuNP-modified acrylic substrate was transparent with a red tint. A heat treatment promoted the formation of amide bonds between carboxy groups on the AuNPs and amino groups on the substrate surface. These covalent bonds stabilized the immobilized AuNPs and the resulting substrate was resistant to washing with acid and thiol-containing solutions. The surface density of AuNPs was controlled by the surface density of amino groups on the substrate surface, which was in turn controlled by the dip-coating in the functional polymer solution. We attempted to immobilize functional biomolecules on the AuNPs-functionalized plastic surface by two different approaches. An enzyme (horseradish peroxidase) was successfully immobilized on the AuNPs through amide formation and 5′-thiolated DNA was also immobilized on the AuNPs through S–Au interactions. These chemistries allow for simultaneous immobilization of two different kinds of biomolecules on a plastic substrate without loss of their functional properties.

We propose a novel approach to stably immobilize gold nanoparticles (AuNPs) on a plastic substrate and demonstrate that the modified substrate is also capable of immobilizing biomolecules.

Interfacial Tension Measurements in Microfluidic Quasi-Static Extensional Flows

Droplet microfluidics provides a versatile tool for measuring interfacial tensions between two immiscible fluids owing to its abilities of fast response, enhanced throughput, portability and easy manipulations of fluid compositions, comparing to conventional techniques. Purely homogeneous extension in the microfluidic device is desirable to measure the interfacial tension because the flow field enables symmetric droplet deformation along the outflow direction. To do so, we designed a microfluidic device consisting of a droplet production region to first generate emulsion droplets at a flow-focusing area. The droplets are then trapped at a stagnation point in the cross junction area, subsequently being stretched along the outflow direction under the extensional flow. These droplets in the device are either confined or unconfined in the channel walls depending on the channel height, which yields different droplet deformations. To calculate the interfacial tension for confined and unconfined droplet cases, quasi-static 2D Darcy approximation model and quasi-static 3D small deformation model are used. For the confined droplet case under the extensional flow, an effective viscosity of the two immiscible fluids, accounting for the viscosity ratio of continuous and dispersed phases, captures the droplet deformation well. However, the 2D model is limited to the case where the droplet is confined in the channel walls and deforms two-dimensionally. For the unconfined droplet case, the 3D model provides more robust estimates than the 2D model. We demonstrate that both 2D and 3D models provide good interfacial tension measurements under quasi-static extensional flows in comparison with the conventional pendant drop method.

Nanophotonic-Carbohydrate Lab-on-a-Microneedle for Rapid Detection of Human Cystatin C in Finger-Prick Blood

Miniaturized total analysis systems, for the rapid detection of disease biomarkers, with features including high biomarker sensitivity, selectivity, biocompatibility, and disposability, all at low cost are of profound importance in the healthcare sector. Within this frame of reference, we developed a lab-on-a-carbohydrate-microneedle biodevice by integrating localized surface plasmon resonance (LSPR) paper-based substrates with biocompatible microneedles of high aspect ratio (>60:1 length:width). These microneedles are completely fabricated with carbohydrate (maltose) and further coated with poly lactic-co-glycolic acid (PLGA), which together serves the purpose of fluid channels. The porous nature of PLGA, in addition to drawing blood by capillary action, filters out the whole blood, allowing only the blood plasma to reach the biorecognition layer of the developed biodevice. While the use of maltose provides biocompatibility to the microneedle, the axial compression and transverse load analysis revealed desired mechanical strength of the microneedle, with mechanical failure occurring at 11N and 9 N respectively for the compressive and transverse load. For a proof-of-principle demonstration, the developed biodevice is validated for its operational features by direct detection of cystatin C in finger-prick blood and up to a concentration of 0.01 μg/mL in buffered conditions using the LSPR technique. Furthermore, by changing the biorecognition layer, the use of the developed needle can be extended to other disease biomarkers, and therefore the innovation presented in this work represents a hallmark in the state of the art of lab-on-a-chip biodevices.

Exploiting the signatures of nanoplasmon-exciton coupling on proton sensitive insulator-semiconductor devices for drug discovery applications

Multimodal sensing methods have a great promise in biosensing applications as they can measure independently several properties that characterise the biomolecular interaction to be detected as well as providing inherent on-chip validation of the sensing signals. This work describes the mechanisms of a concept of insulator-semiconductor field-effect devices coupled with nanoplasmonic sensing as a promising technology, which can be used for a wide range of analytical sensing applications. The developed method involves coupling of the localized surface plasmons (LSPs) within gold nanoparticles (AuNPs) and excitons within pH sensitive silicon nitride (Si3N4) nanofilms for screening inhibitors of kinase, which constitute an important class of chemotherapy drugs. In parallel to this optical sensing, the pH sensitivity of silicon nitride is used to detect the release of protons associated with kinase activity. By changing the insulator and AuNPs characteristics, this work demonstrates the nanoplasmonic-exciton effects taking place, enabling the developed platform to be used for screening kinase inhibitors and as a dual mode electro-optical biosensor for routine bio/chemical sensing applications.

Plasma-Assisted Large-Scale Nanoassembly of Metal-Insulator Bioplasmonic Mushrooms

Large-scale plasmonic substrates consisting of metal-insulator nanostructures coated with a biorecognition layer can be exploited for enhanced label-free sensing by utilizing the principle of localized surface plasmon resonance (LSPR). Most often, the uniformity and thickness of the biorecognition layer determine the sensitivity of plasmonic resonances as the inherent LSPR sensitivity of nanomaterials is limited to 10-20 nm from the surface. However, because of time-consuming nanofabrication processes, there is limited work on both the development of large-scale plasmonic materials and the subsequent surface functionalizing with biorecognition layers. In this work, by exploiting properties of reactive ions in an SF₆ plasma environment, we are able to develop a nanoplasmonic substrate containing ∼10⁶/cm² mushroom-like structures on a large-sized silicon dioxide substrate (i.e., 2.5 cm by 7.5 cm). We further investigate the underlying mechanism of the nanoassembly of gold on glass inside the plasma environment, which can be expanded to a variety of metal-insulator systems. By incorporating a novel microcontact printing technique, we deposit a highly uniform biorecognition layer of proteins on the nanoplasmonic substrate. The bioplasmonic assays performed on these substrates achieve a limit of detection of 10^-17 g/mL (∼66 zM) for biomolecules such as antibodies (∼150 kDa). Our simple nanofabrication procedure opens new opportunities in fabricating versatile bioplasmonic materials for a wide range of biomedical and sensing applications.

A Large-Area Nanoplasmonic Sensor Fabricated by Rapid Thermal Annealing Treatment for Label-Free and Multi-Point Immunoglobulin Sensing

Immunoglobulins are important biomarkers to evaluate the immune status or development of infectious diseases. To provide timely clinical treatments, it is important to continuously monitor the level of multiple immunoglobulins. Localized surface plasmon resonance (LSPR)-based nanoplasmonic sensors have been demonstrated for multiplex immunoglobulins detection. However, the sensor fabrication process is usually slow and complicated, so it is not accessible for large-area and batch fabrication. Herein, we report a large-area (2 cm × 2 cm) nanofabrication method using physical vapor deposition followed by a rapid thermal annealing treatment. To optimize the sensor performance, we systematically characterized three fabrication conditions, including (1) the deposition thickness; (2) the maximum annealing temperature, and (3) the annealing time. The corresponding absorbance spectrum profile and surface morphology of the nanostructures were observed by a UV-VIS spectrometer and atomic force microscopy. We then tested the sensitivity of the sensor using a glucose solution at different concentrations. The results showed that the sensor with 10 nm gold deposition thickness under 5-min 900 °C rapid thermal annealing can achieve the highest sensitivity (189 nm RIU⁻¹). Finally, we integrated this nanoplasmonic sensor with a microchannel and a motorized stage to perform a 10-spot immunoglobulin detection in 50 min. Based on its real-time, dynamic and multi-point analyte detection capability, the nanoplasmonic sensor has the potential to be applied in high-throughput or multiplex immunoassay analysis, which would be beneficial for disease diagnosis or biomedical research in a simple and cost-effective platform.