Recent advances in functionalization of plasmonic nanostructures for optical sensing

Investigation of the molecular switching process between spin crossover states of triazole complexes as basis for optical sensing applications

With the advent of the first laser sources and suitable detectors, optical sensor applications immediately also came into focus. During the last decades, a huge variety of optical sensor concepts were developed, yet the forecast for the future application potential appears even larger. In this context, the development of new sensor probes at different scales down to the atomic or molecular level open new avenues for research and development. We investigated an iron based triazole molecular spin-crossover complex changing its absorption characteristics significantly by varying environmental parameters such as humidity, temperature, magnetic or electric field, respectively, with respect to its suitability for a new class of versatile molecular sensor probes. Hereby, besides the investigation of synthesized pure bulk material using different analyzing methods, we also studied amorphous micro particles which were applied in or onto optical waveguide structures. We found that significant changes of the reflection spectra can also be obtained after combining the particles with different types of optical waveguides.The obtained results demonstrate the suitability of the material complex for a broad field of future sensor applications.

UV-light-responsive Ag/TiO2/PVA nanocomposite for photocatalytic degradation of Cr, Ni, Zn, and Cu heavy metal ions

The rapid growth of industrialization has led to the uncontrolled pollution of the environment, and rapid action is needed. This study synthesized Ag/TiO2/polyvinyl alcohol (PVA) nano photocatalyst for promising light-derived photocatalytic removal of heavy metal ions. The design of experiment (DOE) was used to study the effect of important factors (pH, reaction time, and photocatalyst dosage) to maximize the final performance of the photocatalyst. In the optimized condition, the Ag/TiO2/PVA nano-photocatalyst removed more than 94% of Cr6+ in 180 min, and the efficiency was more than 70% for Cu2+, Zn2+, and Ni2+ metal ions. The adsorption of the heavy metal ions on the photocatalyst was described well with the Langmuir isotherm, while the pseudo-second-order linear kinetic model fitted with the experimental data. The nano-photocatalyst's stability was confirmed after maintaining its performance for five successive runs. The enhanced photocatalytic activity for the heavy metal ions removal can be attributed to the presence of metallic silver nanoparticles (electron transfer and plasmonic fields mechanisms) and PVA, which delayed the recombination of electron-hole. The synthesized ternary Ag/TiO2/PVA nano-photocatalyst showed promising performance for the elimination of heavy metal ions and can be used for environmental remediation purposes.

Surface Bragg gratings of proteins patterned on integrated waveguides for (bio)chemical analysis

The incorporation of biomacromolecules onto silicon waveguiding microstructures constitutes a growing trend that pushes towards compact and miniaturized biosensing systems. This paper presents the integration of one-dimensional periodic nanostructures of proteins on the surface of micrometric silicon waveguides for transducing binding events between biomacromolecules. The study demonstrates this new bioanalytical principle by experimental results and theoretical calculations, and proves that rib waveguides (1--1.6-µm width) together with protein gratings (495--515-nm period) display suitable spectral responses for this optical biosensing system. Protein assemblies of bovine serum albumin are fabricated on the surface of silicon nitride waveguides, characterized by electron microscopy, and their response is measured by optical frequency domain reflectometry along the fabrication process and the subsequent stages of the biorecognition assays. Detection and quantification limits of 0.3 and 3.7 µg·mL-1, respectively, of specific antibodies are inferred from experimental dose-response curves. Among other interesting features, the results of this study point towards new miniaturized and integrated sensors for label-free bioanalysis.

Green Synthesis of Gold, Silver, Copper, and Magnetite Particles Using Poly(tartaric acid) Simultaneously as Coating and Reductant

Poly(tartaric acid) is a relatively recently described polymer that can be easily synthesized and scaled up from a readily available renewable material (tartaric acid). This article demonstrates its use in a green synthesis of gold nanoparticles, silver nanoparticles, copper particles, and magnetite nanoparticles. In this case poly(tartaric acid) acts both as a reductant and as a coating agent. To our knowledge this is the first green synthesis of several different types of nanoparticles using only one reagent (polytartrate) as both reductant and coating. The resulting particles were analyzed by XRD, TEM/SEM, EDX, FTIR, DLS, zeta-potential, XPS, and UV/VIS spectroscopy. Preliminary studies of the thermal behavior of mixtures of different types of particles with poly(tartaric acid) were also conducted. The obtained particles show different sizes depending on the material, and the coating allows for better dispersibility as well as potential further functionalization, making them potentially useful also for other applications, besides the inclusion in polymer composites.

Size-Controlled Silver Nanoparticles Supported by Pyrolytic Carbon from Microcrystalline Cellulose

A facile method was developed for preparing size-controlled silver nanoparticles supported by pyrolytic carbon from microcrystalline cellulose (MCC). The pyrolysis of cellulose-AgNO3 mixture caused the oxidation of cellulose, resulting in carboxyl groups to which silver ions can bind firmly and act as nuclei for the deposition of silver nanoparticles. The structure and properties of the obtained nanocomposite were characterized by using a scanning electron microscope (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), Fourier transform infrared (FT-IR) and X-ray diffraction (XRD). The results suggest that silver nanoparticles were integrated successfully and dispersed uniformly in the pyrolytic carbon matrix. The average particle size varied between 20 nm and 100 nm in correlation to the dose of silver nitrate and temperature of pyrolysis. The products showed high electric conductivity and strong antimicrobial activity against Escherichia coli (E. coli).

Theoretical Study of Gold Nanoparticles Randomly Dispersed on a Dielectric/Gold Substrate

We theoretically study random arrangements of cylindrical gold nanoparticles (NPs) deposited on a dielectric/gold substrate. We use two methods, namely the Finite Element Method (FEM) and the Coupled Dipole Approximation (CDA) method. The FEM is increasingly used to analyze the optical properties of NPs, but calculations for arrangements containing a large number of NPs have a high computational cost. On the contrary, the CDA has the advantage to drastically reduce the computation time and the memory demand compared to the FEM. Nevertheless, as the CDA involves modeling each NP as a single electric dipole through the polarizability tensor of a spheroidal-shaped NP, it may be an insufficiently accurate method. Therefore, the main purpose of this article is to verify the validity of using the CDA in order to analyze such a kind of nanosystems. Finally, we capitalize on this methodology to draw some tendencies between statistics of NPs' distributions and the plasmonic properties.

Detection of Glucose Based on Noble Metal Nanozymes: Mechanism, Activity Regulation, and Enantioselective Recognition

Glucose monitoring is essential to evaluate the degree of glucose metabolism disorders. The enzymatic determination has been the most widely used method in glucose detection because of its high efficiency, accuracy, and sensitivity. Noble metal nanomaterials (NMs, i.e., Au, Ag, Pt, and Pd), inheriting their excellent electronic, optical, and enzyme-like properties, are classified as noble metal nanozymes (NMNZs). As the NMNZs are often involved in two series of reactions, the oxidation of glucose and the chromogenic reaction of peroxide, here the chemical mechanism by employing NMNZs with glucose oxidase (GOx) and peroxidase (POD) mimicking activities is briefly summarized first. Subsequently, the regulation strategies of the GOx-like, POD-like and tandem enzyme-like activities of NMNZs are presented in detail, including the materials, size, morphology, composition, and the reaction condition of the representative NMs. In addition, in order to further mimic the enantioselectivity of enzyme, the design of NMNZs with enantioselective recognition of d-glucose and l-glucose by using different chiral compounds (DNA, amino acids, and cyclodextrins) and molecular imprinting is further described in this review. Finally, the feasible solutions to the existing challenges and a vision for future development possibilities are discussed.

Biofunctionalization of Multiplexed Silicon Photonic Biosensors

Silicon photonic (SiP) sensors offer a promising platform for robust and low-cost decentralized diagnostics due to their high scalability, low limit of detection, and ability to integrate multiple sensors for multiplexed analyte detection. Their CMOS-compatible fabrication enables chip-scale miniaturization, high scalability, and low-cost mass production. Sensitive, specific detection with silicon photonic sensors is afforded through biofunctionalization of the sensor surface; consequently, this functionalization chemistry is inextricably linked to sensor performance. In this review, we first highlight the biofunctionalization needs for SiP biosensors, including sensitivity, specificity, cost, shelf-stability, and replicability and establish a set of performance criteria. We then benchmark biofunctionalization strategies for SiP biosensors against these criteria, organizing the review around three key aspects: bioreceptor selection, immobilization strategies, and patterning techniques. First, we evaluate bioreceptors, including antibodies, aptamers, nucleic acid probes, molecularly imprinted polymers, peptides, glycans, and lectins. We then compare adsorption, bioaffinity, and covalent chemistries for immobilizing bioreceptors on SiP surfaces. Finally, we compare biopatterning techniques for spatially controlling and multiplexing the biofunctionalization of SiP sensors, including microcontact printing, pin- and pipette-based spotting, microfluidic patterning in channels, inkjet printing, and microfluidic probes.

Intrinsic Dual-Emitting Carbon Quantum-Dot-Based Selective Ratiometric Fluorescent Mercaptopurine Detection

Mercaptopurine (6-MP), an immunosuppressive drug, has been widely prescribed for treating leukemia and autoimmune diseases. The level of the 6-MP drug in body fluids is of great interest due to the severe health problems related to its overdose. This study used a facile microwave preparation route to synthesize carbon quantum dots (CQDs) using glutathione and formamide as carbon sources. The obtained monodispersed quantum dots showed dual fluorescence emission with a sensitive affinity toward the 6-MP drug. The sensor's response was optimized by tuning the temperature, pH, and volume ratio of the probe. The prepared ratiometric fluorescence method showed accurate measurements for determining mercaptopurine in aqueous solutions in the concentration range of 1.4-7.6 mg L^-1 with the limit of detection of 1.3 mg L^-1. The sensor's performance was assessed in complex solutions, human urine, and human plasma sample and recovery values in the range of 88-127% were obtained. The reliable dual fluorometric sensor showed promising results for 6-MP determination and potential application for the determination of other chemical and biochemical species.

Selective colorimetric detection of Cr(VI) using starch-stabilized silver nanoparticles and application for chromium speciation

The selective quantitation of Cr(VI) was developed through colorimetric detection using starch-stabilized silver nanoparticles (starch-AgNPs). The detection of Cr(VI) was based on oxidative degradation of starch-AgNPs as the reduction of UV-Vis signal and the distinctive color change from yellow to colorless of starch-AgNPs were observed. To achieve the highest sensitivity by this method, pH 3, and only 1 min were required for the determination of Cr(VI). For analytical performances, two linear ranges of 0-6 µM and 10-80 µM with limit of detection of 0.93 µM (48 µg/L) and 11.57 µM (0.60 mg/L) were obtained, respectively. The developed colorimetric method was combined with inductively coupled plasma optical emission spectroscopy (ICP-OES) for the speciation analysis of chromium. The concentration of Cr(III) was calculated by subtraction of Cr(VI) from the total chromium concentration determined by ICP-OES. A satisfactory accuracy and precision based on the AOAC guidelines also proved that this simple and rapid sensor was successfully applied for speciation of chromium in real water samples.

Simple SPR-based colorimetric sensor to differentiate Mg2+ and Ca2+ in aqueous solutions

L-tryptophan functionalized AgNPs were successfully fabricated using a one-pot synthesis method and assessed as a colorimetric probe for rapid and accurate determination of Mg²⁺ ions. The developed sensor showed a selective response towards Mg²⁺ with no interference from Ca²⁺ in the wide concentration range of 1-200 µM. The sensor's response was optimized in the pH range of 9-10, which can be attributed to the protonation of amine groups and their interaction with Mg²⁺ ions. The stability and selectivity of the sensor were examined in different salt (NaCl) and other metal ions, respectively. The L-tryptophan-AgNPs sensor detected Mg²⁺ with the limit of detection of 3 µM, which is way lower than the concentration range of magnesium in human serum (0.75-1.05 mM). The recovery values of the developed sensor were in the range of 96-102% for the determination of Mg²⁺ in urine samples. The obtained performances proved the potential application of the developed sensor for clinical diagnostic of Mg²⁺ ions where an accurate and rapid response is needed.

Controllable release ratiometric fluorescent sensor for hyaluronidase via the combination of Cu2+-Fe-N-C nanozymes and degradable intelligent hydrogel

As a popular controllable-released carrier, intelligent hydrogels are often used in drug delivery and disease therapeutics. Meanwhile, benefit from the mimic-enzyme activity performance, Fe-N-C nanozymes have been widely used in sensing and analysis. However, the combination of intelligent hydrogels with specific degradability and Fe-N-C nanozymes with enhanced activity in one system to achieve controllable and sensitive detection is rare. Herein, we combine intelligent hydrogel with mimic peroxidase activity enhanced Fe-N-C nanozymes to construct a ratiometric fluorescence probe for sensitive detection of hyaluronidase (HAase). The modification of copper ions has been proved to enhance the mimic enzyme activity of Fe-N-C nanozymes greatly. Cu²⁺ modified Fe-N-C nanozymes were embedded in hyaluronic acid hydrogel. In the presence of HAase, the HA hydrogel structure was hydrolyzed and released Cu²⁺-Fe-N-C nanozymes gradually. The released Cu²⁺-Fe-N-C nanozymes are used to catalyze the hydrogen peroxide system so that o-phenylenediamine is oxidized to orange fluorescent 2, 3-diaminophenolazine (DAP). Due to the electrostatic interaction, the fluorescence resonance energy transfer can occur between the negatively charged copper nanoclusters emitted by 430 nm and the positively charged DAP emitted by 560 nm. The activity of HAase was monitored according to the ratio of fluorescence intensity at 560 nm and 430 nm (F560/F430). The linear range of this method is 0-10.0 U/ml and the detection limit is 0.43 U/mL (S/N = 3). This strategy has been further applied to biological samples successfully.

Disposable biosensors based on metal nanoparticles

The coronavirus disease2019 (COVID-19) pandemic has highlighted the need for disposable biosensors that can detect viruses in infected patients quickly due to fast response and also at a low cost.The present review provides an overview of the applications of disposable biosensors based on metal nanoparticles in enzymatic and non-enzymatic sensors with special reference to glucose and H₂O₂, immunosensors as well as genosensors (DNA biosensors in which the recognized event consists of the hybridization reaction)for point-of-care diagnostics. The disposable biosensors for COVID19 have also been discussed.

Graphitic Carbon Nitride as a Platform for the Synthesis of Silver Nanoclusters

Graphitic carbon nitride (CN) synthetized by the thermal polycondensation of melamine at 550 °C for 4 h was further exfoliated by heating at 500 °C for 3 h. Silver cations were adsorbed on the exfoliated graphitic carbon nitride (CNE) and then reduced by sodium borohydride forming silver nanoclusters (NCs) with a size of less than 1 nm. The NCs were located on the CNE surface and did not change the CNE properties except for its pore size distribution and thereby specific surface area (SSA). The Ag NCs were able to collect the photoinduced electrons of CNE and thus reduce their recombination with the holes. It was also documented by the increase in the CNE photocatalytic activity in terms of the degradation of antibiotic Ofloxacin. This study demonstrates the ability of CNE to serve as a platform for a simple and fast synthesis of Ag NCs without any stabilizing compounds.

Nanoparticle Determination in Water by LED-Excited Surface Plasmon Resonance Imaging

The increasing popularity of nanoparticles in many applications has led to the fact that these persistent materials pollute our environment and threaten our health. An online sensor system for monitoring the presence of nanoparticles in fresh water would be highly desired. We propose a label-free sensor based on SPR imaging. The sensitivity was enhanced by a factor of about 100 by improving the detector by using a high-resolution camera. This revealed that the light source also needed to be improved by using LED excitation instead of a laser light source. As a receptor, different self-assembled monolayers have been screened. It can be seen that the nanoparticle receptor interaction is of a complex nature. The best system when taking sensitivity as well as reversibility into account is given by a dodecanethiol monolayer on the gold sensor surface. Lanthanide-doped nanoparticles, 29 nm in diameter and with a similar refractive index to the most common silica nanoparticles were detected in water down to 1.5 µg mL−1. The sensor can be fully regenerated within one hour without the need for any washing buffer. This sensing concept is expected to be easily adapted for the detection of nanoparticles of different size, shape, and composition, and upon miniaturization, suitable for long-term applications to monitor the quality of water.