Effect of Surface Coverage of Gold Nanoparticles on the Refractive Index Sensitivity in Fiber-Optic Nanoplasmonic Sensing

Bioengineered amyloid peptide for rapid screening of inhibitors against main protease of SARS-CoV-2

The coronavirus disease 2019 (COVID-19) caused by severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) has evoked a worldwide pandemic. As the emergence of variants has hampered the neutralization capacity of currently available vaccines, developing effective antiviral therapeutics against SARS-CoV-2 and its variants becomes a significant challenge. The main protease (Mpro) of SARS-CoV-2 has received increased attention as an attractive pharmaceutical target because of its pivotal role in viral replication and proliferation. Here, we generated a de novo Mpro-inhibitor screening platform to evaluate the efficacies of Mpro inhibitors based on Mpro cleavage site-embedded amyloid peptide (MCAP)-coated gold nanoparticles (MCAP-AuNPs). We fabricated MCAPs comprising an amyloid-forming sequence and Mpro-cleavage sequence, mimicking in vivo viral replication process mediated by Mpro. By measuring the proteolytic activity of Mpro and the inhibitory efficacies of various drugs, we confirmed that the MCAP-AuNP-based platform was suitable for rapid screening potential of Mpro inhibitors. These results demonstrated that our MCAP-AuNP-based platform has great potential for discovering Mpro inhibitors and may accelerate the development of therapeutics against COVID-19.

Competitive Assay for the Ultrasensitive Detection of Organophosphate Pesticides Based on a Fiber-Optic Particle Plasmon Resonance Biosensor and an Acetylcholinesterase Binding Peptide

An acetylcholinesterase (AChE) binding-based biosensor was developed for the ultrasensitive detection of organophosphate (OP) pesticides. The biosensor integrates the technique based on fiber-optic particle plasmon resonance detection and a synthetic AChE binding peptide conjugated with gold nanoparticles on the optical fiber surface via an AChE competitive binding assay. The OP pesticides present in the solution hinder the binding of AChE to the peptide on the biosensor by competing for the binding sites present in AChE. The limit of detection obtained for parathion using this method was observed to be 0.66 ppt (2.3 pM). This method shows a wide linear dynamic range of 6 orders. Furthermore, the use of the AChE binding peptide in the biosensor can better discriminate OPs against carbamates by using only a single biosensor. The practical application of this method was tested using spiked samples, which yielded good recovery and reproducibility. The spiked sample required minimal pretreatment before analysis; hence, this biosensor may also be used in the field.

Controlled Temporal Release of Serum Albumin Immobilized on Gold Nanoparticles

Proteins adsorbed to gold nanoparticles (AuNPs) form bioconjugates and are critical to many emerging technologies for drug delivery, diagnostics, therapies, and other biomedical applications. A thorough understanding of the interaction between the immobilized protein and AuNP is essential for the bioconjugate to perform as designed. Here, we explore a correlation between the number of solvent-accessible thiol groups on a protein and the protein desorption rate from the AuNP surface in the presence of a competing protein. The chemical modification of human serum albumin (HSA) was carried out to install additional free thiols using Traut's reagent and create a library of HSA analogues by tailoring the molar excess of the Traut's reagent. We pre-adsorbed HSA variants onto the AuNP surface, and the resulting bioconjugates were then exposed to IgG antibody, and protein exchange was monitored as a function of time. We found that the rate of HSA displacement from the AuNP correlated with the experimentally measured number of accessible free thiol groups. Additionally, bioconjugates were synthesized using thiolated analogues of bovine serum albumin (BSA) and suspended in serum as a model for a complex sample matrix. Similarly, desorption rates with serum proteins were modulated with solvent-accessible thiols on the immobilized protein. These results further highlight the key role of Au-S bonds in the formation of protein-AuNP conjugates and provide a pathway to systematically control the number of free thiols on a protein, enabling the controlled release of protein from the surface of AuNP.

Quantitative and amplification-free detection of SOCS-1 CpG methylation percentage analyses in gastric cancer by fiber optic nanoplasmonic biosensor

A new innovative approach is essential for early and effective diagnosis of gastric cancer, using promoter hypermethylation of the tumor suppressor, SOCS-1, that is frequently inactivated in human cancers. We have developed an amplification-free fiber optic nanoplasmonic biosensor for detecting DNA methylation of the SOCS-1 human genome. The method is based on the fiber optic nanogold-linked sorbent assay of PCR-free DNA from human gastric tumor tissue and cell lines. We designed a specific DNA probe fabricated on the fiber core surface while the other probe is bioconjugated with gold nanoparticles in free form to allow percentage determination and differentiating the methylated and unmethylated cell lines, further demonstrating the SOCS-1 methylation occurs in cancer patients but not in normal cell lines. The observed detection limit is 0.81 fM for methylated DNA, and the detection time is within 15 min. In addition, our data were significantly correlated to the data obtained from PCR-based pyrosequencing, and yet with superior accuracy. Hence our results provide new insight to the quantitative evaluation of methylation status of the human genome and can act as an alternative to PCR with a great potential.

Integration of Power-Free and Self-Contained Microfluidic Chip with Fiber Optic Particle Plasmon Resonance Aptasensor for Rapid Detection of SARS-CoV-2 Nucleocapsid Protein

The global pandemic of COVID-19 has created an unrivalled need for sensitive and rapid point-of-care testing (POCT) methods for the detection of infectious viruses. For the novel coronavirus SARS-CoV-2, the nucleocapsid protein (N-protein) is one of the most abundant structural proteins of the virus and it serves as a useful diagnostic marker for detection. Herein, we report a fiber optic particle plasmon resonance (FOPPR) biosensor which employed a single-stranded DNA (ssDNA) aptamer as the recognition element to detect the SARS-CoV-2 N-protein in 15 min with a limit of detection (LOD) of 2.8 nM, meeting the acceptable LOD of 10⁶ copies/mL set by the WHO target product profile. The sensor chip is a microfluidic chip based on the balance between the gravitational potential and the capillary force to control fluid loading, thus enabling the power-free auto-flowing function. It also has a risk-free self-contained design to avoid the risk of the virus leaking into the environment. These findings demonstrate the potential for designing a low-cost and robust POCT device towards rapid antigen detection for early screening of SARS-CoV-2 and its related mutants.

Ultrasensitive and Rapid Detection of N-Terminal Pro-B-Type Natriuretic Peptide (NT-proBNP) Using Fiber Optic Nanogold-Linked Immunosorbent Assay

The N-terminal pro-brain natriuretic peptide (NT-proBNP) is considered an important blood biomarker for heart failure. Herein, we report about a fiber optic nanogold-linked immunosorbent assay (FONLISA) method for the rapid, sensitive, and low-cost detection of NT-proBNP. The method is based on a sandwich immunoassay approach that uses two monoclonal NT-proBNP antibodies, a capture antibody (Ab^C), and a detection antibody (Ab^D). Ab^D is conjugated to a free gold nanoparticle (AuNP) to form the free AuNP@Ab^D conjugate, and Ab^C is immobilized on an unclad segment of an optical fiber. The detection of analyte (A), in this case NT-proBNP, is based on the signal change due to the formation of an AuNP@Ab^D–A–Ab^C complex on the fiber core surface, where a green light transmitted through the optical fiber will decrease in intensity due to light absorption by AuNPs via the localized surface plasmon resonance effect. This method provides a wide linear dynamic range of 0.50~5000 pg·mL⁻¹ and a limit of detection of 0.058 pg·mL⁻¹ for NT-proBNP. Finally, the method exhibits good correlation (r = 0.979) with the commercial central laboratory-based electrochemiluminescent immunoassay method that uses a Roche Cobas e411 instrument. Hence, our method is potentially a suitable tool for point-of-care testing.

Design of an unclad single-mode fiber-optic biosensor based on localized surface plasmon resonance by using COMSOL Multiphysics 5.1 finite element method

This study proposed an unclad optical fiber biosensor based on the localized surface plasmon resonance phenomenon and operating at 650 nm using COMSOL Multiphysics 5.1 finite element method (FEM). Gold nanoparticles (50 nm thickness) were coated on the middle portion of the unclad fiber. Air, water, blood plasma, liver tissue, colon tissue, and pentanol (C5H11OH) were used as analytical layers with 3 µm. The sensor serves as a theoretical foundation for experimental research. The blood plasma had the highest sensitivity with a sensitivity of 10,638.297 nm/RIU and a resolution of 9.410-6RIU. The proposed sensor is a promising candidate for a low-cost, simple-geometry biochemical sensing solution.

Selective Detection of an Infection Biomarker by an Osteo-Friend Scaffold: Development of a Multifunctional Artificial Bone Substitute

Developments in three-dimensional (3D) printing technologies have led to many potential applications in various biomedical fields, especially artificial bone substitutes (ABSs). However, due to the characteristics of artificial materials, biocompatibility and infection remain issues. Here, multifunctional ABSs have been designed to overcome these issues by the inclusion of a biochemical modality that allows simultaneous detection of an infection biomarker by osteo-friend 3D scaffolds. The developed multifunctional scaffolds consist of calcium-deficient hydroxyapatite (CDHA), which has a similar geometric structure and chemical composition to human bone, and gold nanoparticles (Au NPs), which assists osteogenesis and modulates the fluorescence of labels in their microenvironment. The Au NPs were subsequently conjugated with fluorescent dye-labeled probe DNA, which allowed selective interaction with a specific target biomarker, and the fluorescent signal of the dye was temporally quenched by the Au NP-derived Förster resonance energy transfer (FRET). When the probe DNA unfolded to bind to the target biomarker, the fluorescence signal was recovered due to the increased distance between the dye and Au NPs. To demonstrate this sensing mechanism, a microbial oligonucleotide was selected as a target biomarker. Consequently, the multifunctional scaffold simultaneously facilitated osteogenic proliferation and the detection of the infection biomarker.

Dual-functional gold-iron oxide core-satellite hybrid nanoparticles for sensitivity enhancement in biosensors via nanoplasmonic and preconcentration effects

A common strategy to improve the sensitivity of a biosensor for the detection of a low abundance analyte is to preconcentrate the analyte molecules before detection. A dual-functional gold-iron oxide core-satellite hybrid nanoparticle structure is proposed in this work to overcome the drawbacks of traditional sample pretreatment methods and the methods using non-magnetic nanomaterials for sample pretreatment. The new dual-functional hybrid nanoparticle structure can simultaneously serve as a signal reporter of a biorecognition event and a preconcentrator of a target at an extremely low concentration in a nanoplasmonic biosensor. By utilizing a fiber optic nanogold-linked sorbent assay in the fiber optic particle plasmon resonance (FOPPR) biosensor and an arbitrary DNA sequence as a target, we have demonstrated that the use of the new hybrid nanoparticle structure with magnetic preconcentration improves the limit of detection (LOD) for the DNA by 18 times as compared to the same method without magnetic preconcentration, so that the LOD for detecting the DNA can be as low as 2.6 fM. The new hybrid nanoparticle structure is easy to prepare and its use in the high-sensitivity and low-cost FOPPR biosensor provides vast opportunities in point-of-care applications.

Enhancing photoelectrochemical water splitting with plasmonic Au nanoparticles

The water-based renewable chemical energy cycle has attracted interest due to its role in replacing existing non-renewable resources and alleviating environmental issues. Utilizing the semi-infinite solar energy source is the most appropriate way to sustain such a water-based energy cycle by producing and feeding hydrogen and oxygen. For production, an efficient photoelectrode is required to effectively perform the photoelectrochemical water splitting reaction. For this purpose, appropriately engineered nanostructures can be introduced into the photoelectrode to enhance light-matter interactions for efficient generation and transport of charges and activation of surface chemical reactions. Plasmon enhanced photoelectrochemical water splitting, whose performance can potentially exceed classical efficiency limits, is of great importance in this respect. Plasmonic gold nanoparticles are widely accepted nanomaterials for such applications because they possess high chemical stability, efficiently absorb visible light unlike many inorganic oxides, and enhance light-matter interactions with localized plasmon relaxation processes. However, our understanding of the physical phenomena behind these particles is still not complete. This review paper focuses on understanding the interfacial phenomena between gold nanoparticles and semiconductors and provides a summary and perspective of recent studies on plasmon enhanced photoelectrochemical water splitting using gold nanoparticles.

Gap-Dependent Surface-Enhanced Raman Scattering (SERS) Enhancement Model of SERS Substrate-Probe Combination Using a Polyelectrolyte Nanodroplet as a Distance Controller

The development of surface-enhanced Raman scattering (SERS) biosensor platforms based on the sandwich combination of an SERS substrate and Raman reporter coated gold nanoparticle (AuNP) labeled with antibody has been widely performed for highly sensitive detection of biomolecules. The size of biomolecules located between these SERS-active materials dictates the sensitivity enhancement of the sensor. However, no suitable molecular size is provided. In this study, we report the gap-dependent SERS enhancement model using the combination of two SERS-active materials of 2D arrays of gold core-silver shell nanoparticles (Au@Ag core-shell NPs) as SERS-active substrates and mercaptobenzoic acid (MBA)-labeled AuNPs as SERS probes. The distance between these two materials is finely tuned using layer-by-layer assembled polyelectrolyte multilayer films. The morphology of the polyelectrolyte spacer is controlled into a droplet nanostructure, which is assumed to have a comparable shape with globular biomolecules. The well-controlled height or thickness of polyelectrolyte nanodroplet was achieved by changing number of deposition cycles. By increasing the thickness of the polyelectrolyte nanodroplet, MBA SERS intensities gradually decreased until at 40 nm-thick nanodroplet film and maintained afterward. This spacer thickness defined the limit of plasmonic coupling effect from this SERS probe-substrate combination. The SERS enhancement capability of this model was compared to conventional SERS immunoassay using three different antigen-antibody complex sizes of prostate-specific antigen, carcinoembryonic antigen, and carbohydrate antigen 19-9. Good agreement of the limitation of plasmon coupling as a function of the distance between the SERS substrate-probe combination using this developed model and SERS immunoassay was found. The finding provides valuable guidelines for immune-system selection in SERS immunosensors based on SERS substrate-probe combination.

MutS protein-based fiber optic particle plasmon resonance biosensor for detecting single nucleotide polymorphisms

A new biosensing method is presented to detect gene mutation by integrating the MutS protein from bacteria with a fiber optic particle plasmon resonance (FOPPR) sensing system. In this method, the MutS protein is conjugated with gold nanoparticles (AuNPs) deposited on an optical fiber core surface. The target double-stranded DNA containing an A and C mismatched base pair in a sample can be captured by the MutS protein, causing increased absorption of green light launching into the fiber and hence a decrease in transmitted light intensity through the fiber. As the signal change is enhanced through consecutive total internal reflections along the fiber, the limit of detection for an AC mismatch heteroduplex DNA can be as low as 0.49 nM. Because a microfluidic chip is used to contain the optical fiber, the narrow channel width allows an analysis time as short as 15 min. Furthermore, the label-free and real-time nature of the FOPPR sensing system enables determination of binding affinity and kinetics between MutS and single-base mismatched DNA. The method has been validated using a heterozygous PCR sample from a patient to determine the allelic fraction. The obtained allelic fraction of 0.474 reasonably agrees with the expected allelic fraction of 0.5. Therefore, the MutS-functionalized FOPPR sensor may potentially provide a convenient quantitative tool to detect single nucleotide polymorphisms in biological samples with a short analysis time at the point-of-care sites.

Preparation and Application of Metal Nanoparticals Elaborated Fiber Sensors

In recent years, surface plasmon resonance devices (SPR, or named plamonics) have attracted much more attention because of their great prospects in breaking through the optical diffraction limit and developing new photons and sensing devices. At the same time, the combination of SPR and optical fiber promotes the development of the compact micro-probes with high-performance and the integration of fiber and planar waveguide. Different from the long-range SPR of planar metal nano-films, the local-SPR (LSPR) effect can be excited by incident light on the surface of nano-scaled metal particles, resulting in local enhanced light field, i.e., optical hot spot. Metal nano-particles-modified optical fiber LSPR sensor has high sensitivity and compact structure, which can realize the real-time monitoring of physical parameters, environmental parameters (temperature, humidity), and biochemical molecules (pH value, gas-liquid concentration, protein molecules, viruses). In this paper, both fabrication and application of the metal nano-particles modified optical fiber LSPR sensor probe are reviewed, and its future development is predicted.

A fiber optic nanoplasmonic biosensor for the sensitive detection of ampicillin and its analogs

A novel optical immunosensor for the screening of ampicillin (Amp) residues has been developed. The biosensor is based on fiber optic particle plasmon resonance detection and uses an enhancement method called as fiber optic nanogold-linked immunosorbent assay (FONLISA) for the sensitive detection of antibiotics. A commercial antibody which had a higher affinity for ampicillin than for other β-lactam antibiotics was chosen. A surface competitive binding assay was used in which a fixed concentration of antibiotic-conjugated gold nanoparticles (AuNPs) competes with free unlabeled antibiotic molecules to measure the amount of binding with antibody molecules immobilized on an optical fiber. The synthesis of the 11-mercaptoundecanoic acid (MUA)-ampicillin conjugate facilitates the attachment of the Amp molecules to AuNPs via MUA which acts as a linker between them. This AuNP-Amp conjugate was then used for the detection of β-lactam antibiotics. The practical limit of detection obtained for Amp was 0.74 ppb (7.4 × 10^-10 g/mL) which is lower than the recommended maximum residue limit (MRL) for β-lactams. The method also shows a wide linear range of 4 orders. Its applicability to the determination of ampicillin in spiked milk samples has been demonstrated with good recovery and reproducibility. Graphical abstract.

Controlled Silanization: High Molecular Regularity of Functional Thiol Groups on Siloxane Coatings

A comparative study on deposition and molecular regularity of two organosilanes, i.e., commercially available (3-mercaptopropyl)trimethoxysilane (MPTMS) and newly developed mercaptopropylsilatrane (MPS), was conducted in this work. MPTMS and MPS were applied to modify silicon surfaces to characterize their deposition kinetics, surface morphology, thickness, and elemental composition and the reactivity of thiol end groups based on gold-thiol and thiol-ene chemistries. MPS possesses a tricyclic caged structure and a transannular N → Si dative bond, making it chemically stable and controllable to avoid fast hydrolysis and aggregation in solution. The results indicate that MPS allows faster deposition and better formation of thin and homogeneous films than MPTMS. More importantly, the functional thiol groups on MPS coatings enable immobilization of a large amount of gold nanoparticles and effective thiol-ene photopolymerization with zwitterionic sulfobetaine acrylamide. Postmodification on silanized surfaces with MPS endows excellent plasmonic and antifouling properties, potentially leading to valuable applications to biosensing and biomaterials. The work demonstrated the feasibility and applicability of the functional silatrane molecule for surface silanization in a controlled manner.

Fiber Optic Particle Plasmon Resonance Biosensor for Label-Free Detection of Nucleic Acids and Its Application to HLA-B27 mRNA Detection in Patients with Ankylosing Spondylitis

We developed a label-free, real-time, and highly sensitive nucleic acid biosensor based on fiber optic particle plasmon resonance (FOPPR). The biosensor employs a single-strand deoxyoligonucleotides (ssDNA) probe, conjugated to immobilized gold nanoparticles on the core surface of an optical fiber. We explore the steric effects on hybridization affinity and limit of detection (LOD), by using different ssDNA probe designs and surface chemistries, including diluent molecules of different lengths in mixed self-assembled monolayers, ssDNA probes of different oligonucleotide lengths, ssDNA probes in different orientations to accommodate target oligonucleotides with a hybridization region located unevenly in the strand. Based on the optimized ssDNA probe design and surface chemistry, we achieved LOD at sub-nM level, which makes detection of target oligonucleotides as low as 1 fmol possible in the 10-μL sensor chip. Additionally, the FOPPR biosensor shows a good correlation in determining HLA-B27 mRNA, in extracted blood samples from patients with ankylosing spondylitis (AS), with the clinically accepted real-time reverse transcription-polymerase chain reaction (RT-PCR) method. The results from this fundamental study should guide the design of ssDNA probe for anti-sense sensing. Further results through application to HLA-B27 mRNA detection illustrate the feasibility in detecting various nucleic acids of chemical and biological relevance.

Fiber Optic Particle Plasmon Resonance-Based Immunoassay Using a Novel Multi-Microchannel Biochip

A novel multi-microchannel biochip fiber-optic particle plasmon resonance (FOPPR) sensor system for the simultaneous detection of multiple samples. The system integrates a novel photoelectric system, a lock-in module, and an all-in-one platform incorporating optical design and mechanical design together to improve system stability and the sensitivity of the FOPPR sensor. The multi-microchannel FOPPR biochip has been developed by constructing a multi-microchannel flow-cell composed of plastic material to monitor and analyze five samples simultaneously. The sensor system requires only 30 μL of sample for detection in each microchannel. Moreover, the total size of the multi-microchannel FOPPR sensor chip is merely 40 mm × 30 mm × 4 mm; thus, it is very compact and cost-effective. The analysis was based on calibration curves obtained from real-time sensor response data after injection of sucrose solution, streptavidin and anti-dinitrophenyl (anti-DNP) antibody of known concentrations over the chips. The results show that the multi-microchannel FOPPR sensor system not only has good reproducibility (coefficient of variation (CV) < 10%), but also excellent refractive index resolution (6.23 ± 0.10 × 10⁻⁶ refractive index unit (RIU)). The detection limits are 2.92 ± 0.28 × 10⁻⁸ g/mL (0.53 ± 0.01 nM) and 7.48 ± 0.40 × 10⁻⁸ g/mL (0.34 ± 0.002 nM) for streptavidin and anti-DNP antibody, respectively.