Enzymatic reaction modulated gold nanorod end-to-end self-assembly for ultrahigh sensitively colorimetric sensing of cholinesterase and organophosphate pesticides in human blood

Plasmonic nanoparticle sensors: current progress, challenges, and future prospects

Plasmonic nanoparticles (NPs) have played a significant role in the evolution of modern nanoscience and nanotechnology in terms of colloidal synthesis, general understanding of nanocrystal growth mechanisms, and their impact in a wide range of applications. They exhibit strong visible colors due to localized surface plasmon resonance (LSPR) that depends on their size, shape, composition, and the surrounding dielectric environment. Under resonant excitation, the LSPR of plasmonic NPs leads to a strong field enhancement near their surfaces and thus enhances various light-matter interactions. These unique optical properties of plasmonic NPs have been used to design chemical and biological sensors. Over the last few decades, colloidal plasmonic NPs have been greatly exploited in sensing applications through LSPR shifts (colorimetry), surface-enhanced Raman scattering, surface-enhanced fluorescence, and chiroptical activity. Although colloidal plasmonic NPs have emerged at the forefront of nanobiosensors, there are still several important challenges to be addressed for the realization of plasmonic NP-based sensor kits for routine use in daily life. In this comprehensive review, researchers of different disciplines (colloidal and analytical chemistry, biology, physics, and medicine) have joined together to summarize the past, present, and future of plasmonic NP-based sensors in terms of different sensing platforms, understanding of the sensing mechanisms, different chemical and biological analytes, and the expected future technologies. This review is expected to guide the researchers currently working in this field and inspire future generations of scientists to join this compelling research field and its branches.

Ratiometric fluorescence and colorimetric strategies for assessing activity of butyrylcholinesterase in human serum using g-C3N4 nanosheets, silver ion and o-phenylenediamine

Ratiometric fluorescence and colorimetric strategies for detecting activity of butyrylcholinesterase (BChE) in human serum were developed by using g-C3N4 nanosheets, silver ion (Ag+) and o-phenylenediamine (OPD) as chromogenic agents. The oxidation-reduction reaction of OPD and Ag+ generates 2,3-diaminophenazine (oxOPD). Under exciation at 370 nm, g-C3N4 nanosheets and oxOPD emit fluorescence at 440 nm (F440) and 560 nm (F560), respectively. Additionally, oxOPD exhibits quenching ability towards g-C3N4 nanosheets via photoinduced electron transfer (PET) process. Thiocholine (TCh), as a product of BChE-catalyzed hydrolysis reaction of butylthiocholine iodide (BTCh), can coordinate with Ag+ intensively, and consequently diminish the amount of free Ag+ in the testing system. Less amount of free Ag+ leads to less production of oxOPD, resulting in less fluorescence quenching towards g-C3N4 nanosheets as well as less fluorescence emission of oxOPD. Therefore, by using g-C3N4 nanosheets and oxOPD as fluorescence indicators, the intensity ratio of their fluorescence (F440/F560) was calculated and employed to evaluate the activity of BChE. Similarly, the color variation of oxOPD indicated by the absorbance at 420 nm (ΔA420) was monitored for the same purpose. These strategies were validated to be sensitive and selective for detecting BChE activity in human serum, with limits of detection (LODs) of 0.1 U L-1 for ratiometric fluorescence mode and 0.7 U L-1 for colorimetric mode.

Nanoclusterzyme for Dual Colorimetric Sensings: A Case Study on [Au14 (Dppp)5 I4 ]2

The enzymatic activity of atomically precise metal nanoclusters has recently been recognized; however, the number of nanoclusterzymes is very small. Besides, the applications of nanoclusterzyme wait to be explored. Herein, a novel nanoclusterzyme is synthesized and its structure is majorly resolved by single-crystal X-ray diffraction and mass spectrometry, which reveal that the nanocluster consists of an Au₁₃ icosahedron capped by an exterior shell including four I, three Dppp (1,3-bis(diphenylphosphino) propane) ligands, and a rarely reported Dppp-Au-Dppp handle staple, which contributes a lot to the enzyme activity of [Au₁₄ (Dppp)₅ I₄ ]²⁺ nanocluster. The as-obtained nanocluster can catalyze oxygen to O₂ ^•- under visible light irradiation with a specific activity up to 0.182 U·mg^-1 and lead to the blue color of 3,3',5,5'-tetramethylbenzidine (TMB) in both solution and solid states. With the addition of acetylcholinesterase (AChE), the blue color of (Au₁₄  + TMB) solution system disappears due to the nanoclusterzyme activity inhibition, but the further addition of organophosphorus pesticides (OPs) into the above mixture can restore the nanoclusterzyme and recover the blue color. Based on the color turn-off and on, the various nanoclusterzyme-containing systems are used to colorimetrically sense AChE and OPs with the detection limits reaching 0.04 mU·mL^-1 and 0.02 ng·mL^-1 , respectively.

1D Colloidal chains: recent progress from formation to emergent properties and applications

Integrating nanoscale building blocks of low dimensionality (0D; i.e., spheres) into higher dimensional structures endows them and their corresponding materials with emergent properties non-existent or only weakly existent in the individual building blocks. Constructing 1D chains, 2D arrays and 3D superlattices using nanoparticles and colloids therefore continues to be one of the grand goals in colloid and nanomaterial science. Amongst these higher order structures, 1D colloidal chains are of particular interest, as they possess unique anisotropic properties. In recent years, the most relevant advances in 1D colloidal chain research have been made in novel synthetic methodologies and applications. In this review, we first address a comprehensive description of the research progress concerning various synthetic strategies developed to construct 1D colloidal chains. Following this, we highlight the amplified and emergent properties of the resulting materials, originating from the assembly of the individual building blocks and their collective behavior, and discuss relevant applications in advanced materials. In the discussion of synthetic strategies, properties, and applications, particular attention will be paid to overarching concepts, fresh trends, and potential areas of future research. We believe that this comprehensive review will be a driver to guide the interdisciplinary field of 1D colloidal chains, where nanomaterial synthesis, self-assembly, physical property studies, and material applications meet, to a higher level, and open up new research opportunities at the interface of classical disciplines.

Plasmonic Nanomaterials for Colorimetric Biosensing: A Review

In the last few decades, plasmonic colorimetric biosensors raised increasing interest in bioanalytics thanks to their cost-effectiveness, responsiveness, and simplicity as compared to conventional laboratory techniques. Potential high-throughput screening and easy-to-use assay procedures make them also suitable for realizing point of care devices. Nevertheless, several challenges such as fabrication complexity, laborious biofunctionalization, and poor sensitivity compromise their technological transfer from research laboratories to industry and, hence, still hamper their adoption on large-scale. However, newly-developing plasmonic colorimetric biosensors boast impressive sensing performance in terms of sensitivity, dynamic range, limit of detection, reliability, and specificity thereby continuously encouraging further researches. In this review, recently reported plasmonic colorimetric biosensors are discussed with a focus on the following categories: (i) on-platform-based (localized surface plasmon resonance, coupled plasmon resonance and surface lattice resonance); (ii) colloid aggregation-based (label-based and label free); (iii) colloid non-aggregation-based (nanozyme, etching-based and growth-based).

Facile preparation of fluorescent water-soluble non-conjugated polymer dots and fabricating an acetylcholinesterase biosensor

Acetylcholinesterase (AChE) has been demonstrated as a crucial enzyme in the development and treatment of Alzheimer's disease (AD). The present work reported the preparation of high fluorescence emission, water-soluble, non-conjugated polymer dots (NCPDs) via Schiff base reaction, and its self-assembly between hyperbranched poly(ethylenimine) (PEI) and pyrogallol in aqueous solutions. A one-pot method was introduced, which made the preparation process of the NCPDs more convenient, energy-efficient, and environmentally friendly. The mechanism of the inherent fluorescence of NCPDs and its fluorescence properties were investigated. This study, for the first time, explored the application of NCPDs to a nanoquencher biosensing system, discovering the reversible quenching effect of MnO2 nanosheets for NCPDs. Furthermore, the quenching mechanism of MnO2 for NCPDs was demonstrated to be an inner filter effect (IFE). The NCPDs-MnO2 biosensing system showed a broader detection range from 12.3 to 3675 U L-1 for AChE and the limit of detection (LOD) was as low as 4.9 U L-1. The sensing system has been applied to screen AChE inhibitors, and the result of the positive drug was highly consistent with previous studies. The established method showed a promising prospect in screening for leading compounds in new drug discoveries for AD.

A ratiometric electrochemical biosensor via alkaline phosphatase mediated dissolution of nano-MnO2 and Ru(III) redox recycling for the determination of dimethoate

A sensitive and ratiometric electrochemical biosensor was developed for the determination of dimethoate via alkaline phosphatase (ALP) mediated dissolution of nano-MnO₂ and [Ru(NH₃)₆]³⁺(Ru(III)) redox recycling. The electroactive probe Ru(III) was adsorbed on the nano-MnO₂ with the high specific surface area through electrostatic interaction to form the MnO₂-Ru(III) nanocomposite, which was then fixed on the surface of the glassy carbon electrode. When the dimethoate inhibited the catalytic activity of ALP in a homogeneous system, the hydrolysate L-ascorbic acid (AA) produced by ALP hydrolysis of L-ascorbic acid-trisodium 2-phosphate (AAP) decreased. The solution was then incubated with a glassy carbon electrode modified by MnO₂-Ru(III). At this time, only a small amount of MnO₂-Ru(III) was decomposed and Ru(III) was rapidly electroreduced to Ru(II) on the surface of the electrode. The in-situ produced Ru(II) was chemically oxidized back to Ru(III) by Fe(III). The redox recycling of Ru(III) was completed and the Ru(III) reduction current signal was amplified. The process consumed part of Fe(III) to reduce the reduction current signal of Fe(III), and the ratio of the two reduction currents (I_Ru(III)/I_Fe(III)) increased significantly. The I_Ru(III)/I_Fe(III) value increased with the increase of dimethoate concentration in the linear range of 0.01-300 ng mL^-1, and the detection limit was 6.3 pg mL^-1. It has been successfully applied to the determination of dimethoate in oilseed rape and lettuce with a satisfactory result.

Aptamer-engineered gold nanorod driven an absorbance enhanced strategy for sensitive biomacromolecule profiling

Gold nanorods (AuNRs)-based plasmonic biosensor offers new opportunity for quantification of biomacromolecules due to their high designability and low technical demands. However, existing methods for the optical detection of biomacromolecule require the targets to induce the aggregation or etching of AuNRs. This limits the range of targets that can be detected, because molecules at extremely low concentration are difficult to arouse aggregation or etching of AuNRs. Thus, it is still challenge to design a scheme for the biomacromolecules at extremely low concentration which can't arouse aggregation or etching of AuNRs based on their plasmonic property. This study proposes a universal absorbance enhanced strategy for biomacromolecule detection with aptamers engineered AuNRs. The biosensor assay (Apts/AuNRs) is designed through assembly of two aptamers on AuNRs to specified recognize the target biomacromolecules, forming closed-loop conformation based on the proximity-dependent ligation, producing absorbance enhancement in the plasmonic peak of AuNRs. It is interesting that the absorbance enhancement increases gradually with increasing protein concentration within a certain range, whereas no aggregation or etching of AuNRs was observed compared with the typical AuNRs based LSPR sensor. Taking advantage of the excellent near infrared light absorption of AuNRs, Apts/AuNRs could be utilized to detect red protein such as cytochrome C, which exhibited better performance than AuNPs based plasmonic sensor. On this basis, the selectivity detection of cytochrome C with the detection of limit down to picomole level was demonstrated. By changing the type of aptamers on AuNRs, the sensitive and credible method was also utilized for the analysis of telomerase activity in nerve cell lysate. Telomerase activity in 4 × 10⁴ neuroblastoma cell was determined to be about 3.575 U/L, which was close to the result of ELISA kit. Good recovery was achieved using standard samples recovery. This study broadens the scope of AuNRs based plasmonic ‬‬‬‬‬‬property ‬‬‬‬and offer a simple, sensitive and selective strategy for biomacromolecules detection in complexed biofluid.

Gold Nanorods: The Most Versatile Plasmonic Nanoparticles

Gold nanorods (NRs), pseudo-one-dimensional rod-shaped nanoparticles (NPs), have become one of the burgeoning materials in the recent years due to their anisotropic shape and adjustable plasmonic properties. With the continuous improvement in synthetic methods, a variety of materials have been attached around Au NRs to achieve unexpected or improved plasmonic properties and explore state-of-the-art technologies. In this review, we comprehensively summarize the latest progress on Au NRs, the most versatile anisotropic plasmonic NPs. We present a representative overview of the advances in the synthetic strategies and outline an extensive catalogue of Au-NR-based heterostructures with tailored architectures and special functionalities. The bottom-up assembly of Au NRs into preprogrammed metastructures is then discussed, as well as the design principles. We also provide a systematic elucidation of the different plasmonic properties associated with the Au-NR-based structures, followed by a discussion of the promising applications of Au NRs in various fields. We finally discuss the future research directions and challenges of Au NRs.

Recent advancements in the detection of organophosphate pesticides: a review

Organophosphorus pesticides (OPPs) are generally utilized for the protection of crops from pests. Because the use of OPPs in various agricultural operations has expanded dramatically, precise monitoring of their concentration levels has become the critical issue, which will help in the protection of ecological systems and food supply. However, the World Health Organization (WHO) has classified them as extremely dangerous chemical compounds. Taking their immense use and toxicity into consideration, the development of easy, rapid and highly sensitive techniques is necessary. Despite the fact that there are numerous conventional ways for detecting OPPs, the development of portable sensors is required to make routine analysis considerably more convenient. Some of these advanced techniques include colorimetric sensors, fluorescence sensors, molecular imprinted polymer-based sensors, and surface plasmon resonance-based sensors. This review article specifically focuses on the colorimetric, fluorescence and electrochemical sensors. In this article, the sensing strategies of these developed sensors, analytical conditions and their respective limit of detection are compiled.

Recent Advances on Detection of Insecticides Using Optical Sensors

Insecticides are enormously important to industry requirements and market demands in agriculture. Despite their usefulness, these insecticides can pose a dangerous risk to the safety of food, environment and all living things through various mechanisms of action. Concern about the environmental impact of repeated use of insecticides has prompted many researchers to develop rapid, economical, uncomplicated and user-friendly analytical method for the detection of insecticides. In this regards, optical sensors are considered as favorable methods for insecticides analysis because of their special features including rapid detection time, low cost, easy to use and high selectivity and sensitivity. In this review, current progresses of incorporation between recognition elements and optical sensors for insecticide detection are discussed and evaluated well, by categorizing it based on insecticide chemical classes, including the range of detection and limit of detection. Additionally, this review aims to provide powerful insights to researchers for the future development of optical sensors in the detection of insecticides.

Oxidase-like Nanozyme-Mediated Altering of the Aspect Ratio of Gold Nanorods for Breaking through H2O2-Supported Multicolor Colorimetric Assay: Application in the Detection of Acetylcholinesterase Activity and Its Inhibitors

A convenient, fast, and colorful colorimetric platform with high resolution for acetylcholinesterase (AChE) activity and its inhibitors detection based on the regulation of oxidase-like nanozyme-mediated etching of gold nanorods (AuNRs) has been proposed in this work. MnO₂ nanosheets are selected as the nanozyme. Their excellent oxidase-like activity enables the etching process to proceed smoothly without the usage of unstable H₂O₂. When AChE is present, it catalytically hydrolyzes acetylthiocholine (ATCh) to thiocholine (TCh). With high reducing ability, TCh induces the decomposition of MnO₂ nanosheets, causing them to lose their oxidase-like activity. Thus, the etching of AuNRs is hampered. Consequently, with the increasing concentration of AChE, an apparent change in the AuNRs solution color is observed. The proposed platform achieves high-sensitivity detection of AChE (limit of detection = 0.18 mU/mL). Furthermore, the proposed platform also has been demonstrated its applicability for its inhibitors detection. Benefiting from the advantages of convenient and high resolution of visual readout, the proposed platform holds great potential for the detection of AChE and its inhibitors in clinical diagnosis.

Fluorescent Determination of Butyrylcholinesterase Activity and Its Application in Biological Imaging and Pesticide Residue Detection

Butyrylcholinesterase (BChE) is an essential human cholinesterase relevant to liver conditions and neurodegenerative diseases, which makes it a pivotal biomarker of health. It therefore remains challenging and highly desired to elaborate efficient chemical tools for BChE with simple operations and satisfactory working performance. In this work, a background-free detection strategy was built by virtue of the judicious coupling of a specific BChE-enzymatic reaction and in situ cyclization. High sensitivity with a low limit of detection (LOD) of 0.075 μg/mL could be readily achieved from the blank background and the as-produced emissive indicators, and the specific reaction site contributed to the high selectivity over other bio-species even acetylcholinesterase (AChE). In addition to the multifaceted spectral experiments to verify the sensing mechanism, this work assumed comprehensive studies on the application. The bio-investigation ranged from cells to an organism, declaring a noteworthy prospect in disease diagnosis, especially for Alzheimer's disease (AD), a common neurodegenerative disease with over-expressed BChE. Moreover, its excellent work for inhibition efficacy elucidation was also proved with the accuracy IC₅₀ of tacrine for BChE (8.6 nM), giving rise to an expanded application for trace pesticide determination.

One-Pot Preparation of Peptide-Doped Metal-Amino Acid Framework for General Encapsulation and Targeted Delivery

Metal-organic frameworks (MOFs), especially those made by biological molecules (bio-MOFs), have been proved to be prospective candidates for biomedical applications. However, a simple and universal bio-MOF to load different substances for precise targeting is still lacking. In this work, we propose a facile one-pot method to prepare a peptide-doped bio-MOF for general encapsulation and targeted delivery. This bio-MOF is constructed by 9-fluorenylmethyloxycarbonyl-modified histidine (Fmoc-His) as a bridging linker that coordinates with Zn2+ ions, denoted as ZFH. The Fmoc-His-Asp-Gly-Arg peptide (Fmoc-HDGR) can be easily doped into the ZFH structure with different ratios to modulate the targeting ability of ZFH-DGR. Containing both hydrophobic Fmoc and hydrophilic His moieties, this framework is compatible with encapsulating various types of payloads, including hydrophobic chemotherapeutic, hydrophilic protein, and positively/negatively charged inorganic nanoparticles. It has also been proved to be highly biocompatible and stable in circulation, exhibit the capabilities to target ανβ3 integrin overexpressed on tumor cells, and trigger drug release in a low pH microenvironment at the tumor site. As a proof of concept, Doxorubicin (Dox)-loaded ZFH-DGR (ZFH-DGR/Dox) demonstrated high cell selectivity between liver hepatocellular carcinoma (HepG2) cells and normal liver (L02) cells, which express high and low ανβ3 integrin, respectively. This selectivity endows ZFH-DGR/Dox precise treatment and low toxicity in Heps-bearing liver cancer mice. This work develops a de novo approach to construct a peptide-doped bio-MOF system for universal load, precise delivery, and peptide drug combination therapy in the future.

Cell membrane coated smart two-dimensional supraparticle for in vivo homotypic cancer targeting and enhanced combinational theranostics

Development of intelligent and multifunctional nanoparticle for the diagnosis and treatment of cancer has drawn great attention recently. In this work, we design a smart two-dimensional (2D) supraparticle for tumor targeted magnetic resonance imaging (MRI)/photothermal imaging (PTI) and chemo/photothermal therapy (PTT). Methods: The nanoparticle consists of a manganese dioxide (MnO2) nanosheet coated gold nanorod (GNR) core (loading with chemotherapeutics doxorubicin (DOX)), and cancer cell membrane shell (denoted as CM-DOX-GMNPs). Decoration of cell membrane endows the nanoparticle with greatly improved colloidal stability and homotypic cancer cell targeting ability. Once the nanoparticles enter tumor cells, MnO2 nanosheets can be etched to Mn2+ by glutathione (GSH) and acidic hydrogen peroxide (H2O2) in the cytosol, leading to the release of DOX. Meanwhile, stimuli dependent releasing of Mn2+ can act as MRI contrast agent for tumor diagnosis. Illumination with near-infrared (NIR) light, photothermal conversion effect of GNRs can be activated for synergistic cancer therapy. Results:In vivo results illustrate that the CM-DOX-GMNPs display tumor specific MRI/PTI ability and excellent inhibition effect on tumor growth. Conclusion: This bioinspired nanoparticle presents an effective and intelligent approach for tumor imaging and therapy, affording valuable guidance for the rational design of robust theranostics nanoplatform.

Recent advances in gold nanoparticles-based biosensors for food safety detection

Food safety issue remains a challenge worldwide. Common substances in food can pose a great threat to human health including but not limited to food borne-pathogens, heavy metals, mycotoxins, pesticides, herbicides, veterinary drugs, allergens and illegal additives. To develop rapid, low-cost, portable and on-site detection methods of those contaminants and allergens to ensure food safety, gold nanoparticles (AuNPs) of versatile shapes and morphologies such as nanorods, nanoclusters, nanoflowers, nanostars, nanocages, nanobipyramids and nanowires have been employed as probes because they possess extraordinary properties that can be used to design biosensors enabling detecting various contaminants and allergens. By means of surface modification, AuNPs can directly or indirectly sense specific targets based on different mechanisms, such as hydrogen bonds, nucleic acid hybridization, aptamer-target binding, antigen-antibody recognition, enzyme inhibition, and enzyme-mimicking activity. AuNPs can induce a distinct color change from red to blue when they transform from a monodispersed state to an aggregated state in liquid solution, which can be observed by naked eyes. If Raman molecules are functionalized on AuNPs, their aggregation will alter the interparticle distance and induce the surface-enhanced Raman scattering that can be employed for highly sensitive detection. Ultra-small AuNPs such as Au nanoclusters also feature in fluorescence that enable a fluorescent readout. The formats of AuNPs for food safety detection in real world range broadly including but not limited to films, fibers, liquid solutions, tapes, chips and lateral flow strips. In this review, recent applications of AuNPs-based biosensors for food safety detection will be discussed, mainly in the aspect of different contaminants and allergens encountered in food samples.

A highly selective and recyclable sensor for the electroanalysis of phosphothioate pesticides using silver-doped ZnO nanorods arrays

Silver-doped ZnO nanorods (Ag/ZnO) arrays have in-situ grown onto indium tin oxide (ITO) via the one-pot hydrothermal route towards a highly selective and recyclable electroanalysis of phosphothioate pesticides (PTs) with phoxim (Phox) as a model. It was discovered that the Ag/ZnO arrays-modified electrode could obtain a steady and sharp electrochemical output of solid-state Ag/AgCl at a low potential (i.e., 0.12 V). More importantly, the achieved Ag/AgCl signals could decrease selectively induced by sulfide (S)-containing Phox by the specific Cl-S displacement reaction, which would trigger AgCl into non-electroactive Ag-Phox complex. The Ag/ZnO arrays-modified sensors present a linear range from 0.050 to 700.0 μM for the detection of Phox, with a limit of detection down to 0.010 μM. The practical applicability of the developed electroanalysis strategy was successfully employed to detect Phox in the tap water and cabbage samples. Moreover, the photocatalytic performances of the Ag/ZnO arrays were subsequently verified for the degradation of Phox, displaying the higher photocatalytic efficiency than pure ZnO nanorods. Besides, the as-developed sensor can allow for the recyclable detection of Phox by the Ag/ZnO-photocatalyzed removal of Phox after each of the detection cycles. Therefore, the sensors platform based on Ag/ZnO arrays can be expected to have potential for the electrochemical monitoring and photocatalytic degradation of toxic pesticides in the food and environmental fields.

A Bioorthogonally Synthesized and Disulfide-Containing Fluorescence Turn-On Chemical Probe for Measurements of Butyrylcholinesterase Activity and Inhibition in the Presence of Physiological Glutathione

Butyrylcholinesterase (BChE) is a biomarker in human blood. Aberrant BChE activity has been associated with human diseases. Here we developed a fluorescence resonance energy transfer (FRET) chemical probe to specifically quantify BChE activity in serum, while simultaneously discriminating against glutathione (GSH). The FRET chemical probe 11 was synthesized from a key trifunctional bicyclononyne exo-6 and derivatives of 5-(2-aminoethylamino)-1-naphthalenesulfonic acid (EDANS) and 4-[4-(dimethylamino)phenylazo]benzoic acid (DABCYL). EDANS fluorescence visualization and kinetic analysis of 11 in the presence of diverse compounds confirmed the outstanding reactivity and specificity of 11 with thiols. The thiol-dependent fluorescence turn-on property of 11 was attributed to a general base-catalyzed SN2 nucleophilic substitution mechanism and independent of metal ions. Moreover, all thiols, except GSH, reacted swiftly with 11. Kinetic studies of 11 in the presence of covalently modified GSH derivatives corroborated that the steric hindrance of 11 imposing on GSH was the likely cause of the distinguished reactivity. Since GSH commonly interferes in assays measuring BChE activity in blood samples, the 11-based fluorescent assay was employed to directly quantify BChE activity without GSH interference, and delivered a linear range of 4.3–182.2 U L−1 for BChE activity with detection limit of 4.3 U L−1, and accurately quantified serum BChE activity in the presence of 10 μM GSH. Finally, the 11-based assay was exploited to determine Ki of 5 nM for tacrine inhibition on BChE catalysis. We are harnessing the modulated characteristics of 6 to synthesize advanced chemical probes able to more sensitively screen for BChE inhibitors and quantify BChE activity in serum.

Ultrasensitive split-type electrochemical sensing platform for sensitive determination of organophosphorus pesticides based on MnO2 nanoflower-electron mediator as a signal transduction system

Organophosphorus pesticides (OPs) are extensively used worldwide as agrochemicals; however, excess use may threaten the health of humans. Thus, it is an urgent need to develop a sensitive method for determination of OPs. Herein, a simple and sensitive split-type electrochemical method was developed by using MnO₂ nanoflower-electron mediator as a signal transduction element. The MnO₂ nanoflower-electron mediator was synthesized and shows an excellent electrochemical signal attributed to the high specific surface area of MnO₂ nanoflower. Meanwhile, the inhibition of OPs on butyrylcholinesterase (BChE) was carried out in the homogeneous system. In the absence of target molecule, a large number of thiocholines (TCh) were yielded from hydrolysis of acetylthiocholine (ATCh) by BChE. The MnO₂ nanoflower was cracked, and subsequently, multiple electron mediator molecules were released from the platform after treated with TCh, thus decreasing the electrochemical response. Furthermore, the inhibition of OPs on BChE resulted in the reduced generation of TCh, thus inducing the recovery of electrochemical signal. Under the optimal experimental, dichlorvos can be detected in a wide range of 10^-6-10^-10 M, with a detection limit of 3 × 10^-10 M. Moreover, the assay was successfully used to analyze dichlorvos in cucumber juice and pear juice, showing a great promising potential for detecting organophosphorus pesticides in complex samples. Graphical abstract In this assay, a split-type electrochemical biosensor was proposed for the ultrasensitive determination of organophosphorus pesticides based on the MnO₂ nanoflower-electron mediator as an electrochemical signal component.

Thiol inhibition of Hg cold vapor generation in SnCl2/NaBH4 system: A homogeneous bioassay for H2O2/glucose and butyrylcholinesterase/pesticide sensing by atomic spectrometry

Recently, the use of atomic spectrometry (AS) for biochemical analysis has attracted considerable attention due to its high sensitivity, selectivity and anti-interference ability. In this work, we conducted a detailed study on a phenomenon of thiol inhibition of mercury (Hg²⁺) cold vapor generation (CVG) and found _L-cysteine (_L-Cys), glutathione (GSH), dithiothreitol, N-Acetyl-L-cysteine, 3-mercaptopropionic acid, β-mercaptoethanol, and NaI can inhibit the CVG of Hg²⁺, while EDTA has no inhibitory effect. Furthermore, changing the content of -SH can effectively adjust the CVG atomic fluorescence spectrometer (CVG-AFS) signal of Hg²⁺. As as a consequence, an AS-based homogeneous bioassay was constructed by adjusting the oxidation ratio and production quantity of -SH in the system. The quantitative analysis of the system was demonstrated by using AFS as a representative detector. Hydrogen peroxide (H₂O₂) and glucose were used as representative analytes for the validation of Hg²⁺ atomic fluorescence signal turn-off strategy, and butyrylcholinesterase (BChE) as well as parathion (organophosphorus pesticides, OPs) as utilized as representative targets for the signal turn-on strategy. Under optimal experimental conditions, the homogeneous CVG-AFS sensor can be successfully used to detect 3 μM H₂O₂, 30 μM glucose, 0.25 U/L BChE, and 0.4 μg/mL parathion. In addition, the detection results of glucose and BChE in human serum samples agreed well with those obtained by using glucometer and kit, showing the promising potential of this method for practical applications. Therefore, this work provides a perspective for the construction of AS-based homogeneous bioassays and shows great potential for the detection of biomarkers.

A review on colorimetric methods for determination of organophosphate pesticides using gold and silver nanoparticles

This review (with 99 refs.) summarizes the progress that has been made in colorimetric (i.e. spectrophotometric) determination of organophosphate pesticides (OPPs) using gold and silver nanoparticles (NPs). Following an introduction into the field, a first large section covers the types and functions of organophosphate pesticides. Methods for colorimetric (spectrophotometric) measurements including RGB techniques are discussed next. A further section covers the characteristic features of gold and silver-based NPs. Syntheses and modifications of metal NPs are covered in section 5. This is followed by overviews on enzyme inhibition-based assays, aptamer-based assays and chemical (non-enzymatic) assays, and a discussion of specific features of colorimetric assays. Several Tables are presented that give an overview on the wealth of methods and materials. A concluding section addresses current challenges and discusses potential future trends and opportunities. Graphical abstractSchematic representation of organophosphate pesticide determinations based on aggregation of nanoparticles (particular silver or gold nanoparticles). This leads to a color change which can be determined visually and monitored by a red shift in the absorption spectrum.

A Review on Cancer Therapy Based on the Photothermal Effect of Gold Nanorod

Background:

Cancer causes millions of deaths and huge economic losses every year. The currently practiced methods for cancer therapy have many defects, such as side effects, low curate rate, and discomfort for patients.

Objective:

Herein, we summarize the applications of gold nanorods (AuNRs) in cancer therapy based on their photothermal effect-the conversion of light into local heat under irradiation.

Methods:

The recent advances in the synthesis and regulation of AuNRs, and facile surface functionalization further facilitate their use in cancer treatment. For cancer therapy, AuNRs need to be modified or coated with biocompatible molecules (e.g. polyethylene glycol) and materials (e.g. silicon) to reduce the cytotoxicity and increase their biocompatibility, stability, and retention time in the bloodstream. The accumulation of AuNRs in cancerous cells and tissues is due to the high leakage in tumors or the specific interaction between the cell surface and functional molecules on AuNRs such as antibodies, aptamers, and receptors.

Results:

AuNRs are employed not only as therapeutics to ablate tumors solely based on the heat produced under laser that could denature protein and activate the apoptotic pathway, but also as synergistic therapies combined with photodynamic therapy, chemotherapy, and gene therapy to kill cancer more efficiently. More importantly, other materials like TiO2, graphene oxide, and silicon, etc. are incorporated on the AuNR surface for multimodal cancer treatment with high drug loadings and improved cancer-killing efficiency. To highlight their applications in cancer treatment, examples of therapeutic effects both in vitro and in vivo are presented.

Conclusion:

AuNRs have potential applications for clinical cancer therapy.

Discrimination of cysteamine from mercapto amino acids through isoelectric point-mediated surface ligand exchange of β-cyclodextrin-modified gold nanoparticles

A pI-mediated R6G-β-CD@AuNPs system was designed for the first time for the discrimination of CA from GSH/Cys/Hcy in human serum samples.

Janus and core@shell gold nanorod@Cu2−xS supraparticles: reactive site regulation fabrication, optical/catalytic synergetic effects and enhanced photothermal efficiency/photostability

We herein present a reactive site regulation strategy for fabricating well-defined Janus and core@shell gold nanorod-Cu_2−xS dual metal–semiconductor plasmonic supraparticles.

A new [Co21(H2O)4(OH)12]30+ unit-incorporating polyoxotungstate for sensitive detection of dichlorvos

A new [Co₂₁(H₂O)₄(OH)₁₂]³⁰⁺ unit-incorporating polyoxotungstate (denoted as Co25Si6W60) has been hydrothermally synthesized and fully characterized. The unique Co25Si6W60-based biosensing system offers an environmentally-friendly route for detecting dichlorvos.

Assay of serum cholinesterase activity by an amperometric biosensor based on a co-crosslinked choline oxidase/overoxidized polypyrrole bilayer

Based on choline oxidase immobilized by co-crosslinking on an overoxidised polypyrrole modified platinum electrode, a novel electrochemical assay for cholinesterase activity in human serum was developed. The assay was performed by adding an aliquot of cholinesterase standard solution or serum sample to phosphate buffer containing choline or thiocholine ester and measuring the oxidation current of hydrogen peroxide at the rotating modified electrode polarized at +0.7 V vs. SCE. The influence of some experimental parameters such as pH of the assay, mass transport at the electrode, type and concentration of the cholinesterase substrate was studied and optimised. Reversible inhibition of choline oxidase from cholinesterase substrates was evidenced for the first time, which increases in the order of acetylcholine, butyrylcholine and s-butyrylthiocholine. Wide linear range, fast response time and appreciable long-term stability were assured for both acethyl- and butyrylcholinesterase assays. On allowing the polypyrrole layer to efficiently remove interferences from the electroactive compounds in the sample, the present method revealed to be suitable for the detection of butyrylcholinesterase in human serum at activities as low as 0.5 U L^-1. The validation with a reference spectrophotometric method showed no significant differences when human serum samples were analysed.

Generic Assay of Sulfur-Containing Compounds Based on Kinetics Inhibition of Gold Nanoparticle Photochemical Growth

This work describes a new, equipment-free, generic method for the determination of sulfur-containing compounds that is based on their ability to slow down the photoreduction kinetics of gold ions to gold nanoparticles. The method involves tracking the time required for a red coloration to appear in the tested sample, indicative of the formation of gold nanoparticles, and compare the measured time relative to a control sample in the absence of the target analyte. The method is applicable with minimal and simple steps requiring only two solutions (i.e., a buffer and a gold solution), a source of light (UV or visible), and a timer. The method responds to a large variety of sulfur-containing compounds including thiols, thioesters, disulfides, thiophosphates, metal-sulfur bonds, and inorganic sulfur and was therefore applied to the determination of a variety of compounds such as dithiocarbamate and organophosphorous pesticides, biothiols, pharmaceutically active compounds, and sulfides in different samples such as natural waters and wastewater, biological fluids, and prescription drugs. The analytical figures of merit of the method include satisfactory sensitivity (quantitation limits at the low μM levels), good recoveries (from 93 to 109%), and satisfactory reproducibility (from 4.8 to 9.8%). The method is easily adoptable to both laboratory settings and nonlaboratory conditions for quantitative and semiquantitative analysis, respectively, is user-friendly even for the minimally trained user, and can be performed with limited resources at low cost.

Colorimetric Determination of Hypochlorite Based on the Oxidative Leaching of Gold Nanorods

Hypochlorite plays a critical role in killing microorganisms in the water. However, it can also cause cardiovascular diseases, neuron degeneration, and cancer to humans. Although traditional methods feature excellent sensitivity and reliability in detecting hypochlorite, the expensive instruments and strict determination conditions have limited their application in environmental analysis to some extent. Thus, it is necessary and urgent to propose a cheap, facile, and quick analytical assay for hypochlorite. This paper proposes a colorimetric assay for hypochlorite utilizing gold nanorods (AuNRs) as the nanoreactor and color reader. The AuNRs were acquired via a reported seed-mediated method. NaClO with strong oxidation property can cause the etching of gold from the longitudinal tips of AuNRs, which could shorten the aspect ratio of AuNRs, decrease the absorption in the UV–Vis spectrum and also induce the solution color changing from red to pale yellow. Thus, according to the solution color change and the absorbance of longitudinal surface plasmon resonance of AuNRs, we established the calibration curve of NaClO within 0.08 μM to 125 μM (∆Abs = 0.0547 + 0.004 C_NaClO, R² = 0.9631). Compared to traditional method, we obtained the conversion formula between the concentration of residual-chlorine in tap water and the concentration of hypochlorite detected by the proposed colorimetric assay, which is C_{residual-chlorine} = 0.24 C_NaClO. Finally, the real application of the colorimetric assay in tap water was successfully performed, and the accuracy of the colorimetric method can reach from −6.78% to +8.53%.

Redox-Controlled Fluorescent Nanoswitch Based on Reversible Disulfide and Its Application in Butyrylcholinesterase Activity Assay

Butyrylcholinesterase (BChE) mainly contributing to plasma cholinesterase activity is an important indicator for routinely diagnosing liver function and organophosphorus poisoning in clinical diagnosis, but its current assays are scarce and frequently suffer from some significant interference and instability. Herein, we report a redox-controlled fluorescence nanoswtich based on reversible disulfide bonds, and further develop a fluorometric assay of BChE via thiol-triggered disaggregation-induced emission. Thiol-functionalized carbon quantum dots (thiol-CQDs) with intense fluorescence is found to be responsive to hydrogen peroxide, and their redox reaction transforms thiol-CQDs to nonfluorescent thiol-CQD assembly. The thiols inverse this process by a thiol-exchange reaction to turn on the fluorescence. The fluorescence can be reversibly switched by the formation and breaking of disulfide bonds caused by external redox stimuli. The specific thiol-triggered disaggregation-induced emission enables us to assay BChE activity in a fluorescence turn-on and real-time way using butyrylthiocholine iodide as the substrate. As-established BChE assay achieves sufficient sensitivity for practical determination in human serum, and is capable of avoiding the interference from micromolar glutathione and discriminatively quantifying BChE from its sister enzyme acetylcholinesterase. The first design of reversible redox-controlled nanosiwtch based on disulfide expands the application of disulfide chemistry in sensing and clinical diagnostics, and this novel BChE assay enriches the detection methods for cholinesterase activity.

A novel fluorescent aptasensor for ultrasensitive and selective detection of acetamiprid pesticide based on the inner filter effect between gold nanoparticles and carbon dots

A novel fluorescent aptasensor based on the inner filter effect of carbon dots has been proposed for the ultrasensitive and selective detection of acetamiprid pesticide in vegetable samples.

Gold nanoparticle-based colorimetric biosensors

Gold nanoparticles (AuNPs) provide excellent platforms for the development of colorimetric biosensors as they can be easily functionalised, displaying different colours depending on their size, shape and state of aggregation. In the last decade, a variety of biosensors have been developed to exploit the extent of colour changes as nano-particles (NPs) either aggregate or disperse, in the presence of analytes. Of critical importance to the design of these methods is that the behaviour of the systems has to be reproducible and predictable. Much has been accomplished in understanding the interactions between a variety of substrates and AuNPs, and how these interactions can be harnessed as colorimetric reporters in biosensors. However, despite these developments, only a few biosensors have been used in practice for the detection of analytes in biological samples. The transition from proof of concept to market biosensors requires extensive long-term reliability and shelf life testing, and modification of protocols and design features to make them safe and easy to use by the population at large. Developments in the next decade will see the adoption of user friendly biosensors for point-of-care and medical diagnosis as innovations are brought to improve the analytical performances and usability of the current designs. This review discusses the mechanisms, strategies, recent advances and perspectives for the use of AuNPs as colorimetric biosensors.

Highly Sensitive Ratiometric Fluorescent Sensor for Trinitrotoluene Based on the Inner Filter Effect between Gold Nanoparticles and Fluorescent Nanoparticles

In this work, we developed a simple and sensitive ratiometric fluorescent assay for sensing trinitrotoluene (TNT) based on the inner filter effect (IFE) between gold nanoparticles (AuNPs) and ratiometric fluorescent nanoparticles (RFNs), which was designed by hybridizing green emissive carbon dots (CDs) and red emissive quantum dots (QDs) into a silica sphere as a fluorophore pair. AuNPs in their dispersion state can be a powerful absorber to quench CDs, while the aggregated AuNPs can quench QDs in the IFE-based fluorescent assays as a result of complementary overlap between the absorption spectrum of AuNPs and emission spectrum of RFNs. As a result of the fact that TNT can induce the aggregation of AuNPs, with the addition of TNT, the fluorescent of QDs can be quenched, while the fluorescent of CDs would be recovered. Then, ratiometric fluorescent detection of TNT is feasible. The present IFE-based ratiometric fluorescent sensor can detect TNT ranging from 0.1 to 270 nM, with a detection limit of 0.029 nM. In addition, the developed method was successfully applied to investigate TNT in water and soil samples with satisfactory recoveries ranging from 95 to 103%, with precision below 4.5%. The simple sensing approach proposed here could improve the sensitivity of colorimetric analysis by changing the ultraviolet analysis to ratiometric fluorescent analysis and promote the development of a dual-mode detection system.

Monodisperse Dual Plasmonic Au@Cu2-xE (E= S, Se) Core@Shell Supraparticles: Aqueous Fabrication, Multimodal Imaging, and Tumor Therapy at in Vivo Level

We herein report aqueous fabrication of well-defined Au@Cu_2-xE (E = S, Se) core@shell dual plasmonic supraparticles (SPs) for multimodal imaging and tumor therapy at the in vivo level. By means of a modified self-limiting self-assembly based strategy, monodisperse core@shell dual plasmonic SPs, including spherical Au@Cu_2-xS SPs, Au@Cu_2-xSe SPs, and rod-like Au@Cu_2-xS SPs, are reliably and eco-friendly fabricated in aqueous solution. Due to plasmonic coupling from the core and shell materials, the as-prepared hybrid products possess an extremely large extinction coefficient (9.32 L g^-1 cm^-1 for spherical Au@Cu_2-xS SPs) at 808 nm, which endows their excellent photothermal effect. Furthermore, the hybrid core@shell SPs possess the properties of good biocompatibility, low nonspecific interactions, and high photothermal stability. So, they show favorable performances for photoacoustic imaging and X-ray computed tomography imaging as well as photothermal therapy of tumors, indicating their application potentials in biological field.

Plasmon-Based Colorimetric Nanosensors for Ultrasensitive Molecular Diagnostics

Colorimetric detection of target analytes with high specificity and sensitivity is of fundamental importance to clinical and personalized point-of-care diagnostics. Because of their extraordinary optical properties, plasmonic nanomaterials have been introduced into colorimetric sensing systems, which provide significantly improved sensitivity in various biosensing applications. Here we review the recent progress on these plasmonic nanoparticles-based colorimetric nanosensors for ultrasensitive molecular diagnostics. According to their different colorimetric signal generation mechanisms, these plasmonic nanosensors are classified into two categories: (1) interparticle distance-dependent colorimetric assay based on target-induced forming cross-linking assembly/aggregate of plasmonic nanoparticles; and (2) size/morphology-dependent colorimetric assay by target-controlled growth/etching of the plasmonic nanoparticles. The sensing fundamentals and cutting-edge applications will be provided for each of them, particularly focusing on signal generation and/or amplification mechanisms that realize ultrasensitive molecular detection. Finally, we also discuss the challenge and give our future perspective in this emerging field.

Valence States Modulation Strategy for Picomole Level Assay of Hg2+ in Drinking and Environmental Water by Directional Self-Assembly of Gold Nanorods

In this study, we present a valence states modulation strategy for picomole level assay of Hg²⁺ using directional self-assembly of gold nanorods (AuNRs) as signal readout. Hg²⁺ ions are first controllably reduced to Hg⁺ ions by appropriate ascorbic acid, and the reduced Hg⁺ ions react with the tips of the preadded AuNRs and form gold amalgam. Such Hg⁺ decorated AuNRs then end-to-end self-assemble into one-dimensional architectures by the bridging effects of lysine based on the high affinity of NH₂-Hg⁺ interactions. Correspondingly, the AuNRs' longitudinal surface plasmon resonance is gradually reduced and a new broad band appears at 900-1100 nm region simultaneously. The resulting distinctly ratiometric signal output is not only favorable for Hg²⁺ ions detection but competent for their quantification. Under optimal conditions, the linear range is 22.8 pM to 11.4 nM, and the detection limit is as low as 8.7 pM. Various transition/heavy metal ions, such as Pb²⁺, Ti²⁺, Co²⁺, Fe³⁺, Mn²⁺, Ba²⁺, Fe²⁺, Ni²⁺, Al³⁺, Cu²⁺, Ag⁺, and Au³⁺, do not interfere with the assay. Because of ultrahigh sensitivity and excellent selectivity, the proposed system can be employed for assaying ultratrace of Hg²⁺ containing in drinking and commonly environmental water samples, which is difficult to be achieved by conventional colorimetric systems. These results indicate that the present platform possesses specific advantages and potential applications in the assay of ultratrace amounts of Hg²⁺ ions.

Colorimetric biosensor for the assay of paraoxon in environmental water samples based on the iodine-starch color reaction

In this work, a new colorimetric biosensor for the assay of paraoxon was developed via the conventional iodine-starch color reaction and multi-enzyme cascade catalytic reactions. In the presence of acetylcholine chloride, acetylcholinesterase (AChE) and choline oxidase (ChO) catalyzed the formation of H₂O₂, which then activated horseradish peroxidase (HRP) to catalyze the oxidation of KI to produce an iodine-starch color reaction. Upon exposure to paraoxon, the catalytic activity of AChE was inhibited and less H₂O₂ generated, resulting in a decrease in the production of I₂ and a drop in the intensity of solution color. This colorimetric biosensor showed high sensitivity for the assay of paraoxon with a limit of detection 4.7 ppb and was applied for the assay of paraoxon in spiked real samples. By employing the conventional iodine-starch color reaction, this biosensor has the potential of on-site assay of OPs residues in environmental samples.

Oxidase-mimicking activity of ultrathin MnO2 nanosheets in colorimetric assay of acetylcholinesterase activity

In the present study, a novel colorimetric sensing platform was constructed for quantitative detection of acetylcholinesterase (AChE) activity and its inhibitor. Manganese dioxide (MnO₂) nanosheets as an oxidase-mimicking nanomaterial could directly oxidize 3,3',5,5'-tetramethylbenzidine (TMB) into oxTMB without the need for horseradish peroxidase and H₂O₂. When AChE was introduced, acetylthiocholine could be catalytically hydrolyzed to produce thiocholine, which easily triggers the decomposition of MnO₂ nanosheets, causing the decrease of solution absorbance. Owing to the inhibition effect of organophosphorus pesticides, the enzymatic activity was suppressed, preventing the decomposition of MnO₂ and resulting in the increase of absorbance. Under optimal conditions, the colorimetric platform shows sensitive responses to AChE and paraoxon in the range of 0.1-15 mU mL^-1 and 0.001-0.1 μg mL^-1, respectively. The detection limits of AChE and paraoxon reached 35 μU mL^-1 and 1.0 ng mL^-1, respectively. Furthermore, the MnO₂-TMB platform has been used to fabricate test strips for rapid and convenient visual detection of AChE and its inhibitor with highly promising performance.