Development of Optical Differential Sensing Based on Nanomaterials for Biological Analysis

The discrimination and recognition of biological targets, such as proteins, cells, and bacteria, are of utmost importance in various fields of biological research and production. These include areas like biological medicine, clinical diagnosis, and microbiology analysis. In order to efficiently and cost-effectively identify a specific target from a wide range of possibilities, researchers have developed a technique called differential sensing. Unlike traditional "lock-and-key" sensors that rely on specific interactions between receptors and analytes, differential sensing makes use of cross-reactive receptors. These sensors offer less specificity but can cross-react with a wide range of analytes to produce a large amount of data. Many pattern recognition strategies have been developed and have shown promising results in identifying complex analytes. To create advanced sensor arrays for higher analysis efficiency and larger recognizing range, various nanomaterials have been utilized as sensing probes. These nanomaterials possess distinct molecular affinities, optical/electrical properties, and biological compatibility, and are conveniently functionalized. In this review, our focus is on recently reported optical sensor arrays that utilize nanomaterials to discriminate bioanalytes, including proteins, cells, and bacteria.

Dual AuNPs detecting probe enhanced the NanoSPR effect for the high-throughput detection of the cancer microRNA21 biomarker

The microRNA21 (miR-21), a specific tumor biomarker, is crucial for the diagnosis of several cancer types, and investigation of its overexpression pattern is important for cancer diagnosis. Herein, we report a low-cost, rapid, ultrasensitive, and convenient biosensing strategy for the detection of miR-21 using a nanoplasmonic array chip coupled with gold nanoparticles (AuNPs). This sensing platform combines the surface plasmon resonance effect of nanoplasmonics (NanoSPR) and the localized surface plasmon resonance (LSPR) effect, which allows the real-time monitoring of the subtle optical density (OD) changes caused by the variations in the dielectric constant in the process of the hybridization of the target miRNA. Using this method, the miRNA achieves a broad detection range from 100 aM to 1 μM, and with a limit of detection (LoD) of 1.85 aM. Furthermore, this assay also has a single-base resolution to discriminate the highly homologous miRNAs. More importantly, this platform has high throughput characteristics (96 samples can be detected simultaneously). This strategy exhibits more than 86.5 times enhancement in terms of sensitivity compared to that of traditional biosensors.

Amplification-Free, High-Throughput Nanoplasmonic Quantification of Circulating MicroRNAs in Unprocessed Plasma Microsamples for Earlier Pancreatic Cancer Detection

Pancreatic ductal adenocarcinoma (PDAC) is a deadly malignancy that is often detected at an advanced stage. Earlier diagnosis of PDAC is key to reducing mortality. Circulating biomarkers such as microRNAs are gaining interest, but existing technologies require large sample volumes, amplification steps, extensive biofluid processing, lack sensitivity, and are low-throughput. Here, we present an advanced nanoplasmonic sensor for the highly sensitive, amplification-free detection and quantification of microRNAs (microRNA-10b, microRNA-let7a) from unprocessed plasma microsamples. The sensor construct utilizes uniquely designed -ssDNA receptors attached to gold triangular nanoprisms, which display unique localized surface plasmon resonance (LSPR) properties, in a multiwell plate format. The formation of -ssDNA/microRNA duplex controls the nanostructure-biomolecule interfacial electronic interactions to promote the charge transfer/exciton delocalization processes and enhance the LSPR responses to achieve attomolar (10^-18 M) limit of detection (LOD) in human plasma. This improve LOD allows the fabrication of a high-throughput assay in a 384-well plate format. The performance of nanoplasmonic sensors for microRNA detection was further assessed by comparing with the qRT-PCR assay of 15 PDAC patient plasma samples that shows a positive correlation between these two assays with the Pearson correlation coefficient value >0.86. Evaluation of >170 clinical samples reveals that oncogenic microRNA-10b and tumor suppressor microRNA-let7a levels can individually differentiate PDAC from chronic pancreatitis and normal controls with >94% sensitivity and >94% specificity at a 95% confidence interval (CI). Furthermore, combining both oncogenic and tumor suppressor microRNA levels significantly improves differentiation of PDAC stages I and II versus III and IV with >91% and 87% sensitivity and specificity, respectively, in comparison to the sensitivity and specificity values for individual microRNAs. Moreover, we show that the level of microRNAs varies substantially in pre- and post-surgery PDAC patients (n = 75). Taken together, this ultrasensitive nanoplasmonic sensor with excellent sensitivity and specificity is capable of assaying multiple biomarkers simultaneously and may facilitate early detection of PDAC to improve patient care.

Plasmonic and Hybrid Whispering Gallery Mode-Based Biosensors: Literature Review

BACKGROUND

The term "plasmonic" describes the relationship between electromagnetic fields and metallic nanostructures. Plasmon-based sensors have been used innovatively to accomplish different biomedical tasks, including detection of cancer. Plasmonic sensors also have been used in biochip applications and biosensors and have the potential to be implemented as implantable point-of-care devices. Many devices and methods discussed in the literature are based on surface plasmon resonance (SPR) and localized SPR (LSPR). However, the mathematical background can be overwhelming for researchers at times.

OBJECTIVE

This review article discusses the theory of SPR, simplifying the underlying physics and bypassing many equations of SPR and LSPR. Moreover, we introduce and discuss the hybrid whispering gallery mode (WGM) sensing theory and its applications.

METHODS

A literature search in ScienceDirect was performed using keywords such as "surface plasmon resonance," "localized plasmon resonance," and "whispering gallery mode/plasmonic." The search results retrieved many articles, among which we selected only those that presented a simple explanation of the SPR phenomena with prominent biomedical examples.

RESULTS

SPR, LSPR, tilted fiber Bragg grating, and hybrid WGM phenomena were explained and examples on biosensing applications were provided.

CONCLUSIONS

This minireview presents an overview of biosensor applications in the field of biomedicine and is intended for researchers interested in starting to work in this field. The review presents the fundamental notions of plasmonic sensors and hybrid WGM sensors, thereby allowing one to get familiar with the terminology and underlying complex formulations of linear and nonlinear optics.

Individual Plasmonic Nanoprobes for Biosensing and Bioimaging: Recent Advances and Perspectives

With the advent of nanofabrication techniques, plasmonic nanoparticles (PNPs) have been widely applied in various research fields ranging from photocatalysis to chemical and bio-sensing. PNPs efficiently convert chemical or physical stimuli in their local environment into optical signals. PNPs also have excellent properties, including good biocompatibility, large surfaces for the attachment of biomolecules, tunable optical properties, strong and stable scattering light, and good conductivity. Thus, single optical biosensors with plasmonic properties enable a broad range of uses of optical imaging techniques in biological sensing and imaging with high spatial and temporal resolution. This work provides a comprehensive overview on the optical properties of single PNPs, the description of five types of commonly used optical imaging techniques, including surface plasmon resonance (SPR) microscopy, surface-enhanced Raman scattering (SERS) technique, differential interference contrast (DIC) microscopy, total internal reflection scattering (TIRS) microscopy, and dark-field microscopy (DFM) technique, with an emphasis on their single plasmonic nanoprobes and mechanisms for applications in biological imaging and sensing, as well as the challenges and future trends of these fields.

Recent Advances of Silver Nanoparticles in Cancer Diagnosis and Treatment

Background:

Silver nanoparticles (AgNPs) are well-known as a promising antimicrobial material; they have been widely used in many commercial products against pathogenic agents. Despite a growing concern regarding the cytotoxicity, AgNPs still have attracted considerable interest worldwide to develop a new generation of diagnostic tool and effective treatment solution for cancer cells.

Objective:

This paper aims to review the advances of AgNPs applied for cancer diagnosis and treatment.

Methods:

The database has been collected, screened and analysed through up-to-date scientific articles published from 2007 to May 2019 in peer-reviewed international journals.

Results:

The findings of the database have been analysed and divided into three parts of the text that deal with AgNPs in cancer diagnosis, their cytotoxicity, and the role as carrier systems for cancer treatment. Thanks to their optical properties, high conductivity and small size, AgNPs have been demonstrated to play an essential role in enhancing signals and sensitivity in various biosensing platforms. Furthermore, AgNPs also can be used directly or developed as a drug delivery system for cancer treatment.

Conclusion:

The review paper will help readers understand more clearly and systematically the role and advances of AgNPs in cancer diagnosis and treatment.

Revealing Lectin-Sugar Interactions with a Single Au@Ag Nanocube

An individual nanoparticle-based plasmonic nanotechnology was used for real-time monitoring of lectin-sugar interactions, which could be designed as novel plasmonic nanobiosensors for the detection of trace concanavalin A (ConA) with high sensitivity and selectivity. The localized surface plasmon resonance (LSPR) spectra of Au@Ag nanocubes (NCs) are linearly shifted to a long wavelength with an increasing concentration of ConA. In fact, each Au@Ag NC can act as a nanobiosensor for the quantified detection of trace ConA, which enables the miniaturization of the biosensor system to nanoscale. Furthermore, the results demonstrated the perfect biosensing ability with the dual channel of dark-field microscopy images and LSPR spectra. We expect that this nanobiosensor system can provide an alternative important method for monitoring the specific binding of lectin-sugar at a single nanoparticle surface.

Quantitative investigation on the critical thickness of the dielectric shell for metallic nanoparticles determined by the plasmon decay length

Inert dielectric shells coating the surface of metallic nanoparticles (NPs) are important for enhancing the NPs' stability, biocompatibility, and realizing targeting detection, but they impair NPs' sensing ability due to the electric fields damping. The dielectric shell not only determines the distance of the analyte from the NP surface, but also affects the field decay. From a practical point of view, it is extremely important to investigate the critical thickness of the shell, beyond which the NPs are no longer able to effectively detect the analytes. The plasmon decay length of the shell-coated NPs determines the critical thickness of the coating layer. Extracting from the exponential fitting results, we quantitatively demonstrate that the critical thickness of the shell exhibits a linear dependence on the NP volume and the dielectric constants of the shell and the surrounding medium, but only with a small variation influenced by the NP shape where the dipole resonance is dominated. We show the critical thickness increases with enlarging the NP sizes, or increasing the dielectric constant differences between the shell and surrounding medium. The findings are essential for applications of shell-coated NPs in plasmonic sensing.

An Au@Ag nanocube based plasmonic nano-sensor for rapid detection of sulfide ions with high sensitivity

Based on the localized surface plasmon resonance (LSPR) technology, a novel plasmonic nanosensor with high sensitivity and high selectivity was prepared for the detection of trace sulfide ions on an individual Au@Ag nanoparticle.

Individual Au-Nanocube Based Plasmonic Nanoprobe for Cancer Relevant MicroRNA Biomarker Detection

MicroRNA205 (miR-205), as a significant tumor biomarker, is of vital importance for diagnosis of lung cancer and its overexpression patterns have been extensively studied. Here, we report a novel and label-free nanoprobe with high sensitivity and selectivity for miRNA biomarker detection using localized surface plasmon resonance (LSPR) technology on a single DNA modified gold nanocube (AuNC). This method allowed real-time monitoring of the subtle LSPR scattering peak position's change which was aroused by the variation of dielectric constant in the hybridization process of target miRNA with ssDNA modified on the surface of AuNCs. Notably, the limit of detection of the AuNC-ssDNA probe is up to 5 pM in serum sample, and these results showed that the square structure has more superior sensitivity for design and development of nanoprobe for trace lung cancer relevant miRNAs detection. The better sensing ability and stability of LSPR probe on a AuNC provide potential application to developing a high flux biochip in the future.

Few-Layer Graphdiyne Nanosheets Applied for Multiplexed Real-Time DNA Detection

Despite recent progress in 2D nanomaterials-based biosensing, it remains challenging to achieve sensitive and high selective detection. This study develops few-layer graphdiyne (GD) nanosheets (NSs) that are used as novel sensing platforms for a variety of fluorophores real-time detection of DNA with low background and high signal-to-noise ratio, which show a distinguished fluorescence quenching ability and different affinities toward single-stranded DNA and double-stranded DNA. Importantly, for the first time, a few-layer GD NSs-based multiplexed DNA sensor is developed.

Color resolution improvement of the dark-field microscopy imaging of single light scattering plasmonic nanoprobes for microRNA visual detection

Imaging of light scattering plasmonic nanoparticles (PNPs) with the aid of the dark-field microscopy imaging (iDFM) technique has attracted wide attention owing to its high signal-to-noise ratio, but to improve the color resolution and contrast of dark-field microscopy (DFM) images of single light scattering PNPs in a small spectral variation environment is still a challenge. In this study, a new color analytical method for resolving the resolution and contrast in DFM images has been developed and further applied for colorimetric analysis using the digital image processing technique. The color of single light scattering PNP images is automatically coded at first with the hue values of the HSI color model, and then amplified using the MATLAB program even for marginal spectral changes, leading to significant improvement of the color resolution of DFM images and easy detection with the naked eye. As a proof of concept, this method is then applied to distinguish single PNPs with various sizes and to visually detect hepatocellular carcinoma-associated microRNA. As it greatly improved the color resolution of iDFM and its detection sensitivity, this method shows promise to serve as a better alternative for sensitive visual analysis and spectrometer-based spectral analysis.

Single plasmonic nanoparticles as ultrasensitive sensors

In this mini review, we will provide a brief introduction focusing on the current applications of single plasmonic nanoparticle-based sensors using DFM, including the detection of molecules, the real-time monitoring of chemical/electrochemical reactions and the imaging of living cells.

Engineering the hot spots in squared arrays of gold nanoparticles on a silver film

Density of nanoparticle (NP) arrays affects the hot spots distribution and strength in NP-metal film (NP-MF) geometry. In-depth understanding of the variation of electromagnetic (EM) field enhancement with NPs density is essential for wide applications of the NP-MF geometry such as surface-enhanced spectroscopies and enhanced efficiency of optoelectronic devices. Here, we show that the field distribution in the NP array on the metal film is greatly enhanced and confined at the NP-NP junctions for very small horizontal gap (g) between neighboring NPs, whereas the fields at the NP-MF junction are extremely small. When gradually increasing g, the field enhancement at the NP-NP junction decreases, along with the gradually enhanced fields at the NP-MF junction. We show that there is an optimal value of horizontal gap (∼75 nm for 80 nm Au NP array on Ag film with 532 nm normal incidence), indicating that the average field enhancement in NP-MF geometry can be optimized by adjusting the horizontal gap. More importantly, it is found that the EM field enhancement is greatly decreased when g fulfills the requirement to couple the 532 nm incident light into SPPs, because of the interference between the LSPR and the SPPs, which leads to a Fano dip at the incident wavelength of 532 nm.

Citrate-Induced Nanocubes: A Re-Examination of the Role of Citrate as a Shape-Directing Capping Agent for Ag-Based Nanostructures

Seed-mediated syntheses utilizing facet-selective surface passivation provide the necessary chemical controls to direct noble metal nanostructure formation to a predetermined geometry. The foremost protocol for the synthesis of (111)-faceted Ag octahedra involves the reduction of metal ions onto pre-existing seeds in the presence of citrate and ascorbic acid. It is generally accepted that the capping of (111) facets with citrate dictates the shape while ascorbic acid acts solely as the reducing agent. Herein, a citrate-based synthesis is demonstrated in which the presence or absence of ascorbic acid is the shape-determining factor. Reactions are carried out in which Ag(+) ions are reduced onto substrate-immobilized Ag, Au, Pd, and Pt seeds. Syntheses lacking ascorbic acid, in which citrate acts as both the capping and the reducing agent, result in a robust nanocube growth mode able to withstand wide variations in the concentration of reactants, reaction rates, seed material, seed orientation and faceting, pH, and substrate material. If, however, ascorbic acid is included in these syntheses, then the growth mode reverts to one that advances the octahedral geometry. The implication of these results is that citrate, or one of its oxidation products, selectively caps (100) facets, but where this capability is compromised by ascorbic acid.

HSI colour-coded analysis of scattered light of single plasmonic nanoparticles

Single plasmonic nanoparticles (PNPs) analysis with dark-field microscopic imaging (iDFM) has attracted much attention in recent years. The ability for quantitative analysis of iDFM is critical, but cumbersome, for characterizing and analyzing the scattered light of single PNPs. Here, a simple automatic HSI colour coding method is established for coding dark-field microscopic (DFM) images of single PNPs with localized surface plasmon resonance (LSPR) scattered light, showing that hue value in the HSI system can realize accurate quantitative analysis of iDFM and providing a novel approach for quantitative chemical and biochemical imaging at the single nanoparticle level.

An Individual Nanocube-Based Plasmonic Biosensor for Real-Time Monitoring the Structural Switch of the Telomeric G-Quadruplex

Promoted by the localized surface plasmon resonance nanotechnology, a simple and sensitive plasmonic aptamer nanosensor (nanoaptasensor) on an individual Au@Ag core-shell nanocube (Au@Ag NC) has been proposed for real-time monitoring of the formation process of G-quadruplex structures and label-free analysis of potassium ions (K(+) ). In particular, the analysis of the thermodynamic parameters indicates that there are two types of binding states accompanied with a remarkable change of free energy (ΔG) in the sequential folding process of telomere DNA sequence. This nanoaptasensor has raised promising applications in monitoring the dynamic process of the structural switch of the G-quadruplex.

Nanostructure-based surface-enhanced Raman scattering biosensors for nucleic acids and proteins

Detection of nucleic acid and protein targets related to human health and safety has attracted widespread attention. Surface-enhanced Raman scattering (SERS) is a powerful tool for biomarker detection because of its ultrahigh detection sensitivity and unique fingerprinting spectra. In this review, we first introduce the development of nanostructure-based SERS-active substrates and SERS nanotags, which greatly influence the performance of SERS biosensors. We then focus on recent advances in SERS biosensors for DNA, microRNA and protein determination, including label-free, labeled and multiplex analyses as well as in vivo imaging. Finally, the prospects and challenges of such nanostructure-based SERS biosensors are discussed.

SERS-based immunoassay using a core–shell SiO2@Ag immune probe and Ag-decorated NiCo2O4 nanorods immune substrate

A novel sandwich structure consisting of the SiO₂@Ag immune probe and the Ag-decorated NNRs substrate was used to detect AFP and a detection limit is as low as 2.1 fg mL⁻¹.

Hybridization-induced Ag(i) dissociation from an immobilization-free and label-free hairpin DNA: toward a novel electronic monitoring platform

A novel silver ion-assisted hairpin DNA through C–Ag⁺–C coordination chemistry was designed for homogeneous electronic monitoring of HIV DNA on a negatively charged electrode, based on hybridization-induced Ag⁺ dissociation from hairpin DNA.

A cathodic luminol-based electrochemiluminescence biosensor for detecting cholesterol using 3D-MoS2–PANI nanoflowers and Ag nanocubes for signal enhancement

The illustration of the synthetic process of 3D-MoS₂–PANI–AgNCs nanocomposites and the preparation of an ECL biosensor.