Label-Free LSPR Detection of Trace Lead(II) Ions in Drinking Water by Synthetic Poly(mPD-co-ASA) Nanoparticles on Gold Nanoislands

Advanced Integration of Glutathione-Functionalized Optical Fiber SPR Sensor for Ultra-Sensitive Detection of Lead Ions

It is crucial to detect Pb2+ accurately and rapidly. This work proposes an ultra-sensitive optical fiber surface plasmon resonance (SPR) sensor functionalized with glutathione (GSH) for label-free detection of the ultra-low Pb2+ concentration, in which the refractive index (RI) sensitivity of the multimode-singlemode-multimode (MSM) hetero-core fiber is largely enhanced by the gold nanoparticles (AuNPs)/Au film coupling SPR effect. The GSH is modified on the fiber as the sensing probe to capture and identify Pb2+ specifically. Its working principle is that the Pb2+ chemically reacts with deprotonated carboxyl groups in GSH through ligand bonding, resulting in the formation of stable and specific chelates, inducing the variation of the local RI on the sensor surface, which in turn leads to the SPR wavelength shift in the transmission spectrum. Attributing to the AuNPs, both the Au substrates can be fully functionalized with the GSH molecules as the probes, which largely increases the number of active sites for Pb2+ trapping. Combined with the SPR effect, the sensor achieves a sensitivity of 2.32 × 1011 nm/M and a limit of detection (LOD) of 0.43 pM. It also demonstrates exceptional specificity, stability, and reproducibility, making it suitable for various applications in water pollution, biomedicine, and food safety.

Towards Digital Twins of Plasmonic Sensors: Constructing the Complex Numerical Model of a Plasmonic Sensor Based on Hexagonally Arranged Gold Nanoparticles

In this work, we aim to design the digital twin of a plasmonic sensor based on hexagonally arranged ellipsoidal gold nanoparticles fixed to a glass substrate. Based on electron microscopy images of three experimentally realized nanoparticle arrangement types, we constructed numerical models in environments based on finite element method (FEM) and boundary element method (BEM), namely COMSOL Multiphysics for FEM and the MNPBEM Matlab Toolbox for BEM. Models with nonperiodic and periodic boundary conditions with different unit cells were constructed to study the plasmonic behavior of both the single ellipsoidal particles and the hexagonal nanoparticle arrangements. The effect of the geometrical parameters, namely the interparticle distance, the nanoparticle diameter and thickness, on the resulting LSPR peak positions and bulk refractive index sensitivities were studied in detail, also taking into account the effect of the SiO₂ substrate (pillars) under the ellipsoidal particles. We have demonstrated that by optimizing the models, the LSPR peak positions (and sensitivities) can match the experimentally measured values within 1 nm (nm/RIU) precision. The comparison of simulation conditions and the detailed discussion of the effect of the geometrical parameters and used gold dielectric functions on the obtained sensitivities can be very beneficial for the optimization of plasmonic sensor constructions through numerical simulations.

Recent Advancements in Novel Sensing Systems through Nanoarchitectonics

The fabrication of various sensing devices and the ability to harmonize materials for a higher degree of organization is essential for effective sensing systems. Materials with hierarchically micro- and mesopore structures can enhance the sensitivity of sensors. Nanoarchitectonics allows for atomic/molecular level manipulations that create a higher area-to-volume ratio in nanoscale hierarchical structures for use in ideal sensing applications. Nanoarchitectonics also provides ample opportunities to fabricate materials by tuning pore size, increasing surface area, trapping molecules via host-guest interactions, and other mechanisms. Material characteristics and shape significantly enhance sensing capabilities via intramolecular interactions, molecular recognition, and localized surface plasmon resonance (LSPR). This review highlights the latest advancements in nanoarchitectonics approaches to tailor materials for various sensing applications, including biological micro/macro molecules, volatile organic compounds (VOC), microscopic recognition, and the selective discrimination of microparticles. Furthermore, different sensing devices that utilize the nanoarchitectonics concept to achieve atomic-molecular level discrimination are also discussed.

Fabrication of Metal-Insulator-Metal Nanostructures Composed of Au-MgF2-Au and Its Potential in Responding to Two Different Factors in Sample Solutions Using Individual Plasmon Modes

In this paper, metal-insulator-metal (MIM) nanostructures, which were designed to exhibit two absorption peaks within 500-1100 nm wavelength range, were fabricated using magnesium difluoride (MgF2) as the insulator layer. Since the MIM nanostructures have two plasmon modes corresponding to the absorption peaks, they independently responded to the changes in two phases: the surrounding medium and the inside insulator layer, the structure is expected to obtain multiple information from sample solution: refractive index (RI) and molecular interaction between solution components and the insulator layer. The fabricated MIM nanostructure had a diameter of 139.6 ± 2.8 nm and a slope of 70°, and exhibited absorption peaks derived from individual plasmon modes at the 719 and 907 nm wavelengths. The evaluation of the response to surrounding solution component of the MIM nanostructures revealed a linear response of one plasmon mode toward the RI of the surrounding medium and a large blue shift of the other plasmon mode under conditions where glycerol was present at high concentration. From optical simulation and the evaluation of the MgF2 fabricated by deposition, the blue shift was expected to be due to the swelling of MgF2 interacting with the hydroxyl groups abundantly included in the glycerol molecules. The results indicated the individual responses of two plasmon modes in MIM nanostructures toward medium components, and brought the prospect for the simultaneous measurement of multiple elements using two or more plasmon modes.

LSPR-based colorimetric biosensing for food quality and safety

Ensuring consistently high quality and safety is paramount to food producers and consumers alike. Wet chemistry and microbiological methods provide accurate results, but those methods are not conducive to rapid, onsite testing needs. Hence, many efforts have focused on rapid testing for food quality and safety, including the development of various biosensors. Herein, we focus on a group of biosensors, which provide visually recognizable colorimetric signals within minutes and can be used onsite. Although there are different ways to achieve visual color-change signals, we restrict our focus on sensors that exploit the localized surface plasmon resonance (LSPR) phenomenon of metal nanoparticles, primarily gold and silver nanoparticles. The typical approach in the design of LSPR biosensors is to conjugate biorecognition ligands on the surface of metal nanoparticles and allow the ligands to specifically recognize and bind the target analyte. This ligand-target binding reaction leads to a change in color of the test sample and a concomitant shift in the ultraviolet-visual absorption peak. Various designs applying this and other signal generation schemes are reviewed with an emphasis on those applied for evaluating factors that compromise the quality and safety of food and agricultural products. The LSPR-based colorimetric biosensing platform is a promising technology for enhancing food quality and safety. Aided by the advances in nanotechnology, this sensing technique lends itself easily for further development on field-deployable platforms such as smartphones for onsite and end-user applications.

Biosensing Applications Using Nanostructure-Based Localized Surface Plasmon Resonance Sensors

Localized surface plasmon resonance (LSPR)-based biosensors have recently garnered increasing attention due to their potential to allow label-free, portable, low-cost, and real-time monitoring of diverse analytes. Recent developments in this technology have focused on biochemical markers in clinical and environmental settings coupled with advances in nanostructure technology. Therefore, this review focuses on the recent advances in LSPR-based biosensor technology for the detection of diverse chemicals and biomolecules. Moreover, we also provide recent examples of sensing strategies based on diverse nanostructure platforms, in addition to their advantages and limitations. Finally, this review discusses potential strategies for the development of biosensors with enhanced sensing performance.

A label-free lead(II) ion sensor based on surface plasmon resonance and DNAzyme-gold nanoparticle conjugates

Detection of lead(II) (Pb²⁺) ions in water is important for the protection of human health and environment. The growing demand for onsite detection still faces challenges for sensitive and easy-to-use methods. In this work, a novel surface plasmon resonance (SPR) biosensor based on GR-5 DNAzyme and gold nanoparticles (AuNPs) was developed. Thiolated DNAzyme was immobilized on the gold surface of the sensor chip followed by anchoring the substrate-functionalized AuNPs through the DNAzyme-substrate hybridization. The coupling between the localized surface plasmon (LSP) of AuNPs and the surface plasmon polaritons (SPP) on the gold sensor surface was used to improve the sensitivity. The substrate cleavage in the presence of Pb²⁺ ions was catalyzed by DNAzyme, leading to the removal of AuNPs and the diminished LSP-SPP coupling. The optimal detection limit was 80 pM for the sensor fabricated with 1 μM DNAzyme, corresponding to two or three orders of magnitude lower than the toxicity levels of Pb²⁺ in drinking water defined by WHO and USEPA. By tuning the surface coverage of DNAzyme, the sensitivity and dynamic range could be controlled. This sensor also featured high selectivity to Pb²⁺ ions and simple detection procedure. Successful detection of Pb²⁺ ions in groundwater indicates that this method has the prospect in the onsite detection of Pb²⁺ ions in water. Given the variety of AuNPs and metal-specific DNAzymes, this detection strategy would lead to the development of more sensitive and versatile heavy metal sensors. Graphical abstract.

An improved structure-switch aptamer-based fluorescent Pb2+ biosensor utilizing the binding induced quenching of AMT to G-quadruplex

An improved aptamer-based fluorescent Pb²⁺ biosensor utilizing the binding induced quenching of AMT to G-quadruplex has been rationally designed with a LOD of 3.6 nM. The utility of the developed biosensor was demonstrated by the successful detection of Pb²⁺ in real complex clinical samples with satisfactory recovery and good reproducibility.

Label-Free and Enzyme-Free Colorimetric Detection of Pb2+ Based on RNA Cleavage and Annealing-Accelerated Hybridization Chain Reaction

A label-free and enzyme-free colorimetric sensor for rapid detection of Pb²⁺ is reported, which is based on the strategy of DNAzyme-mediated RNA cleavage combined with an annealing-accelerated DNA hybridization chain reaction (HCR). As a trigger DNA, the substrate strand (S_TM) of DNAzyme can initiate HCR effectively. However, when it is cleaved by DNAzyme in the presence of Pb²⁺, the separation of DNA functional domains leads to a serious decrease in HCR efficiency. As a result, the difference in Pb²⁺ concentration converts into the difference of DNA assembly, which eventually leads to the color change of colloidal gold nanoparticles (AuNPs). In this work, a DNA strand (cGR5) completely complementary to the catalytic strand (GR5) of DNAzyme is used to improve the dissociation of S_TM to enhance the HCR efficiency. In addition, the simple operation of DNA annealing is first used to accelerate the HCR process, enabling the Pb²⁺ detection to be completed in about 30 min. As advantages of high sensitivity, good selectivity, strong anti-interference ability, and good practical performance are achieved, it is anticipated that the cheap and simple colorimetric sensor will be helpful for on-site detection of environmental and food samples.

A sensitive LSPR sensor based on glutathione-functionalized gold nanoparticles on a substrate for the detection of Pb2+ ions

A plasmonic probe based on gold nanoparticles (AuNPs) on a solid substrate for the detection of Pb²⁺ was developed.

Novel Biomimic Crystalline Colloidal Array for Fast Detection of Trace Parathion

A novel gold doped inverse opal photonic crystal (IO PC) was successfully fabricated with combination of molecularly imprinted technical for the fast determination of parathion. First, a closest silica array arrangement behaved as the 3D photonic crystal precursors to build the opal photonic crystal (O PC). Second, the parathion-containing polymeric solution with gold nanoparticles was drawn into the 3D array cracks. After polymerization, the well-designed O PC was treated with HF solution for the etching of the silica skeleton. Finally, the template parathion was removed and the Au-MIP IO PCs were obtained. The morphology of SiO₂ and Au NPs was characterized by transmission electron microscopy (TEM), and the eluted influence of the IO PCs was monitored by scanning electron microscopy (SEM). The cross-linking effect was well optimized according to the best spectrum signal of parathion. The as-synthesized Au-MIP IO PCs displayed the specificity toward parathion and the selectivity to other competitive pesticide molecules. The response time was only 5 min, and the parathion could be well detected from real water samples. The recoveries were between 95.5% and 101.5%.