Enzyme-Induced Metallization as a Signal Amplification Strategy for Highly Sensitive Colorimetric Detection of Avian Influenza Virus Particles

Fluorescence/colorimetric dual-signal sensor based on carbon dots and gold nanoparticles for visual quantification of Cr3

A convenient and sensitive dual-signal visualization method is constructed for detection of trivalent chromium ions (Cr3+) based on fluorescent carbon dots (CDs) and glutathione-modified gold nanoparticles (GSH-Au NPs). The fluorescence of CDs can be quenched by GSH-Au NPs due to the inner filter effect. Cr3+ induces aggregation of GSH-Au NPs because of the coordination with GSH on the surface of Au NPs, leading to the red shift of surface plasmon resonance absorption of Au NPs that provides a "turn-on" fluorescence and colorimetric assay for Cr3+. The fluorescence/colorimetric dual signal detection shows high sensitivity for Cr3+ with wide detection linear ranges (0.5-70 μM for fluorescence detection and 2-50 μM for colorimetric detection) and low detection limits (0.31 μM for fluorescence detection and 0.30 μM for colorimetric detection). Besides, the method has high selectivity for Cr3+ and can be used for detection of Cr3+ in lake water and tap water samples, showing great potential for visual detection of environmental Cr3+.

Integrating carbon dots and gold/silver core-shell nanoparticles to achieve sensitive detection of dopamine with fluorometric/colorimetric dual signal

Dopamine (DA) is a potent neuromodulator in the brain that affects a wide range of motivated behaviors. Abnormal concentration of DA is related to a variety of diseases. Hence, it is imperative to establish a rapid and precise method for quantifying DA. In this work, we integrate orange-yellow emissive carbon dots (CDs) with target-induced silver deposition on gold nanoparticles (Au NPs), forming gold/silver core-shell nanoparticles (Au@Ag NPs), to construct a fluorometric and colorimetric dual-signal sensor for sensitive detection of DA. Au NPs and silver ions (Ag+) have minimal effect on the fluorescence of CDs. DA can reduce the silver ions to Ag(0) on the surface of the Au NPs to form a silver shell, resulting in the blue-shift of the absorbance peak from 520 to 416 nm, which overlaps with the excitation spectrum of CDs. As a result, the system color turns from pink to orange-yellow, and the fluorescence of CDs is quenched due to the strong inner filter effect. The linear range of the colorimetry is 0.5-18 μM with a limit of detection (LOD) of 0.41 μM, while the linear range for the fluorometry method is 0.5-14 μM with a LOD of 0.021 μM. This method demonstrates notable advantages including a low detection limit, rapid response time, and straightforward operation in practical samples, showing great potential in biomedical analysis.

Airborne microbes: sampling, detection, and inactivation

The human living environment serves as a habitat for microorganisms and the presence of ubiquitous airborne microbes significantly impacts the natural material cycle. Through ongoing experimentation with beneficial microorganisms, humans have greatly benefited from airborne microbes. However, airborne pathogens endanger human health and have the potential to induce fatal diseases. Tracking airborne microbes is a critical prerequisite for a better understanding of bioaerosols, harnessing their potential advantages, and mitigating associated risks. Although technological breakthroughs have enabled significant advancements in accurately monitoring airborne pathogens, many puzzles about these microbes remain unanswered due to their high variability and environmental diffusibility. Consequently, advanced techniques and strategies for special identification, early warning, and efficient eradication of microbial contamination are continuously being sought. This review presents a comprehensive overview of the research status of airborne microbes, concentrating on the recent advances and challenges in sampling, detection, and inactivation. Particularly, the fundamental design principles for the collection and timely detection of airborne pathogens are described in detail, as well as critical factors for eliminating microbial contamination and enhancing indoor air quality. In addition, future research directions and perspectives for controlling airborne microbes are also suggested to promote the translation of basic research into real products.

Advances in rapid point-of-care virus testing

Outbreaks of viral diseases seriously jeopardize people's health and cause huge economic losses. At the same time, virology provides a new perspective for biology, molecular biology and cancer research, and it is important to study the discovered viruses with potential applications. Therefore, the development of immediate and rapid viral detection methods for the prevention and treatment of viral diseases as well as the study of viruses has attracted extensive attention from scientists. With the continuous progress of science and technology, especially in the field of bioanalysis, a series of new detection techniques have been applied to the on-site rapid detection of viruses, which has become a powerful approach for human beings to fight against viruses. In this paper, the latest research progress of rapid point-of-care detection of viral nucleic acids, antigens and antibodies is presented. In addition, the advantages and disadvantages of these technologies are discussed from the perspective of practical application requirements. Finally, the problems and challenges faced by rapid viral detection methods and their development prospects are discussed.

A point-of-care solid-phase colorimetric sensor based on the enzyme-induced metallization for ALP detection

Alkaline phosphatase (ALP) is a crucial biomarker for clinical diagnosis, which is closely related to the physiological homeostasis regulation process of human body. And the abnormal level of ALP is associated with numerous diseases, such as liver dysfunction, bone diseases, diabetes, and so on. In order to meet the demand of personalized healthcare, it is particularly important to develop a miniaturized point-of-care testing (POCT) device for ALP detection. Herein, a portable solid-phase colorimetric sensor based on enzyme-induced metallization signal amplification strategy was constructed for ALP detection. The AuNPs modified on the glass slides acted as crystal seeds, allowing Ag+ in the solution to be reduced and deposited on the surface of AuNPs, which further formed the gold core and silver shell (Au@Ag) complex and generated visual signals. The visual signals were recorded by a smartphone and quantified using open-source ImageJ software. Under the optimal conditions, the proposed method exhibited a good linear relationship from 2.0 to 16.0 pM, and the detection limit was as low as 0.9 pM. In addition, it was further successfully applied for ALP detection in non-transparent and complex samples (milk, different types of cells). A sensitive, low cost, rapid and convenient solid-phase sensor was developed for ALP detection, which was expected to provide a promising strategy for POCT devices.

Overview on the Development of Alkaline-Phosphatase-Linked Optical Immunoassays

The drive to achieve ultrasensitive target detection with exceptional efficiency and accuracy requires the advancement of immunoassays. Optical immunoassays have demonstrated significant potential in clinical diagnosis, food safety, environmental protection, and other fields. Through the innovative and feasible combination of enzyme catalysis and optical immunoassays, notable progress has been made in enhancing analytical performances. Among the kinds of reporter enzymes, alkaline phosphatase (ALP) stands out due to its high catalytic activity, elevated turnover number, and broad substrate specificity, rendering it an excellent candidate for the development of various immunoassays. This review provides a systematic evaluation of the advancements in optical immunoassays by employing ALP as the signal label, encompassing fluorescence, colorimetry, chemiluminescence, and surface-enhanced Raman scattering. Particular emphasis is placed on the fundamental signal amplification strategies employed in ALP-linked immunoassays. Furthermore, this work briefly discusses the proposed solutions and challenges that need to be addressed to further enhance the performances of ALP-linked immunoassays.

Recent Progress on Optical Biosensors Developed for Nucleic Acid Detection Related to Infectious Viral Diseases

Optical biosensors have many advantages over traditional analytical methods. They enable the identification of several biological and chemical compounds directly, instantly, and without the need of labels. Their benefits include excellent specificity, sensitivity, compact size, and low cost. In this review, the main focus is placed on the nucleic acid-based optical biosensor technologies, including colorimetric, fluorescence, surface plasmon resonance (SPR), Evanescent-Wave Optical, Fiber optic and bioluminescent optical fibre. The fundamentals of each type of biosensor are briefly explained, and particular emphasis has been placed on the achievements which have been gained in the last decade on the field of diagnosis of infectious viral diseases. Concluding remarks concerning the perspectives of further developments are discussed.

Analyte-mediated formation and growth of nanoparticles for the development of chemical sensors and biosensors

The cornerstone of nanomaterial-based sensing systems is the synthesis of nanoparticles with appropriate surface functionalization that ensures their stability and determines their reactivity with organic or inorganic analytes. To accomplish these requirements, various compounds are used as additives or growth factors to regulate the properties of the synthesized nanoparticles and their reactivity with the target analytes. A different rationale is to use the target analytes as additives or growth agents to control the formation and properties of nanoparticles. The main difference is that the analyte recognition event occurs before or during the formation of nanoparticles and it is based on the reactivity of the analytes with the precursor materials of the nanoparticles (e.g., metal ions, reducing agents, and coatings). The transition from the ionic (or molecular) state of the precursor materials to ordered nanostructured assemblies is used for sensing and signal transduction for the qualitative detection and the quantitative determination of the target analytes, respectively. This review focuses on assays that are based on analyte-mediated regulation of nanoparticles' formation and differentiate them from standard nanoparticle-based assays which rely on pre-synthesized nanoparticles. Firstly, the principles of analyte-mediated nanomaterial sensors are described and then they are discussed with emphasis on the sensing strategies, the signal transduction mechanisms, and their applications. Finally, the main advantages, as well as the limitations of this approach, are discussed and compared with assays that rely on pre-synthesized nanoparticles in order to highlight the major advances accomplished with this type of nano-sensors and elucidate challenges and opportunities for further evolving new nano-sensing strategies.

Recent advances in airborne pathogen detection using optical and electrochemical biosensors

The world is currently facing an adverse condition due to the pandemic of airborne pathogen SARS-CoV-2. Prevention is better than cure; thus, the rapid detection of airborne pathogens is necessary because it can reduce outbreaks and save many lives. Considering the immense role of diverse detection techniques for airborne pathogens, proper summarization of these techniques would be beneficial for humans. Hence, this review explores and summarizes emerging techniques, such as optical and electrochemical biosensors used for detecting airborne bacteria (Bacillus anthracis, Mycobacterium tuberculosis, Staphylococcus aureus, and Streptococcus pneumoniae) and viruses (Influenza A, Avian influenza, Norovirus, and SARS-CoV-2). Significantly, the first section briefly focuses on various diagnostic modalities applied toward airborne pathogen detection. Next, the fabricated optical biosensors using various transducer materials involved in colorimetric and fluorescence strategies for infectious pathogen detection are extensively discussed. The third section is well documented based on electrochemical biosensors for airborne pathogen detection by differential pulse voltammetry, cyclic voltammetry, square-wave voltammetry, amperometry, and impedance spectroscopy. The unique pros and cons of these modalities and their future perspectives are addressed in the fourth and fifth sections. Overall, this review inspected 171 research articles published in the last decade and persuaded the importance of optical and electrochemical biosensors for airborne pathogen detection.

Brightness Analysis per Moving Particle: In Situ Analysis of Alkaline Phosphatase in Living Cells

In situ quantitative analysis of enzymes such as phosphatase is important to understand a number of involved biological processes ranging from various metabolisms to signal transduction and cellular regulation. In this paper, a novel in situ measurement strategy was proposed to detect alkaline phosphatase (ALP) activity in different locations within single living cells. The principle is based on the measurement of the resonance light scattering brightness ratio (SBR) per moving nanoparticle that forms in an ALP-related chemical reaction. In the method, a novel resonance light scattering correlation spectroscopy (RLSCS) system was developed using two lasers for illumination or two detection channels. Using the gold nanoparticles (AuNPs) as probes, the Au@Ag nanoparticles (Au@Ag NPs) formed due to the ALP-catalyzed hydrolysis of ascorbic acid 2-phosphate (AAP) and the subsequent reduction-deposition reaction of Ag ions that occurred on the AuNPs. The SBR value per moving particle was determined based on the obtained RLS intensity traces and RLSCS curves. The SBR value was found to be not influenced by the intracellular viscosity and size that was confirmed in the experiments. The linear relation between the SBR and ALP activity was established and applied to detect ALP activity and evaluate the inhibition of different drugs. Finally, the method was successfully used to in situ measure ALP activity within living cells. The method overcomes the shortcoming of conventional methods that lack quantitative analysis and are susceptible to intracellular viscosity.

Advanced Point-of-Care Testing Technologies for Human Acute Respiratory Virus Detection

The ever-growing global threats to human life caused by the human acute respiratory virus (RV) infections have cost billions of lives, created a significant economic burden, and shaped society for centuries. The timely response to emerging RVs could save human lives and reduce the medical care burden. The development of RV detection technologies is essential for potentially preventing RV pandemic and epidemics. However, commonly used detection technologies lack sensitivity, specificity, and speed, thus often failing to provide the rapid turnaround times. To address this problem, new technologies are devised to address the performance inadequacies of the traditional methods. These emerging technologies offer improvements in convenience, speed, flexibility, and portability of point-of-care test (POCT). Herein, recent developments in POCT are comprehensively reviewed for eight typical acute respiratory viruses. This review discusses the challenges and opportunities of various recognition and detection strategies and discusses these according to their detection principles, including nucleic acid amplification, optical POCT, electrochemistry, lateral flow assays, microfluidics, enzyme-linked immunosorbent assays, and microarrays. The importance of limits of detection, throughput, portability, and specificity when testing clinical samples in resource-limited settings is emphasized. Finally, the evaluation of commercial POCT kits for both essential RV diagnosis and clinical-oriented practices is included.

Advanced Nanomaterials for Preparedness Against (Re-)Emerging Viral Diseases

While the coronavirus disease (COVID-19) accounts for the current global pandemic, the emergence of other unknown pathogens, named "Disease X," remains a serious concern in the future. Emerging or re-emerging pathogens continue to pose significant challenges to global public health. In response, the scientific community has been urged to create advanced platform technologies to meet the ever-increasing needs presented by these devastating diseases with pandemic potential. This review aims to bring new insights to allow for the application of advanced nanomaterials in future diagnostics, vaccines, and antiviral therapies, thereby addressing the challenges associated with the current preparedness strategies in clinical settings against viruses. The application of nanomaterials has advanced medicine and provided cutting-edge solutions for unmet needs. Herein, an overview of the currently available nanotechnologies is presented, highlighting the significant features that enable them to control infectious diseases, and identifying the challenges that remain to be addressed for the commercial production of nano-based products is presented. Finally, to conclude, the development of a nanomaterial-based system using a "One Health" approach is suggested. This strategy would require a transdisciplinary collaboration and communication between all stakeholders throughout the entire process spanning across research and development, as well as the preclinical, clinical, and manufacturing phases.

Nanotechnology-based Colorimetric Approaches for Pathogenic Virus Sensing: A review

Fast and inexpensive virus identification protocols are paramount to hinder the further extent of pandemic diseases, minimize economic and social damages, and expedite proper clinical rehabilitation. Until now, various biosensors have been fabricated for the identification of pathogenic particles. But, they offer many difficulties. Nanotechnology resolves these difficulties and offers direct identification of pathogenic species in real-time. Among them, nanomaterial based-colorimetric sensing approach of pathogenic viruses by the naked eye has attracted much awareness because of their simplicity, speed, and low cost. In this review, the latest tendencies and advancements are overviewed in detecting pathogenic viruses using colorimetric concepts. We focus on and reconsider the use of distinctive nanomaterials such as metal nanoparticles, carbon nanotubes, graphene oxide, and conducting polymer to form colorimetric pathogenic virus sensors.

Optical Biosensors for Virus Detection: Prospects for SARS-CoV-2/COVID-19

The recent pandemic of the novel coronavirus disease 2019 (COVID-19) has caused huge worldwide disruption due to the lack of available testing locations and equipment. The use of optical techniques for viral detection has flourished in the past 15 years, providing more reliable, inexpensive, and accurate detection methods. In the current minireview, optical phenomena including fluorescence, surface plasmons, surface-enhanced Raman scattering (SERS), and colorimetry are discussed in the context of detecting virus pathogens. The sensitivity of a viral detection method can be dramatically improved by using materials that exhibit surface plasmons or SERS, but often this requires advanced instrumentation for detection. Although fluorescence and colorimetry lack high sensitivity, they show promise as point-of-care diagnostics because of their relatively less complicated instrumentation, ease of use, lower costs, and the fact that they do not require nucleic acid amplification. The advantages and disadvantages of each optical detection method are presented, and prospects for applying optical biosensors in COVID-19 detection are discussed.

Development of Antibody-Conjugated Gold and Magnetic Nanocomposites for the Colorimetric Detection of Influenza A Virus

A novel approach for the detection of influenza virus is of paramount importance for quick diagnosis and therapy. In this study monoclonal antibody (mAb)-conjugated MNPs/AuNPs were developed to detect the H1N1 virus. MNPs and AuNPs were synthesized and loaded with mAbs. The UV-vis spectra exhibited absorbance at 528 nm. XRD revealed the presence of crystalline particles with various diffraction peaks. FTIR confirmed the occurrence of capping molecules in the synthesized NPs. NP stability was evidenced by zeta measurements. The shape and size (mean size 15 nm) of the NPs were determined using SEM and transmission electron microscopy (TEM). In this study the mAb–AuNPs produced a redshift in the absorption spectrum due to plasmon coupling. The absorption increased when H1N1 concentration increased from 0 to 5.0 ng/mL, with the detection limit being 0.05 ng/mL. The sensitivity of mAb–AuNPs was greater than that of ELISA. Since the mAb-AuNP-based colorimetric immunosensor is simple, cost-effective, and rapidly detects H1N1, it has good prospects in pharmaceuticals and clinical diagnosis.

A colorimetric and electrochemical dual-mode biosensor for thrombin using a magnetic separation technique

In general, protein detection relies primarily on enzyme-linked immunosorbent assays. Here, we constructed a colorimetric and electrochemical dual-mode biosensor for thrombin detection based on the mechanism of aptamer recognition. Magnetic nanobeads (MBs) were used as carriers for separation and enrichment to quickly capture thrombin (TB) in the complex matrix. Also, the combination of MBs and the magnetic electrode array (MEA) effectively avoided the poisoning of the electrode by biological samples. Furthermore, hybridization chain reaction (HCR) was indirectly used to achieve amplification of TB. A large number of horseradish peroxidases (HRPs) were coupled with the amplified long nucleic acid fragments. Based on the color and current response of the substrate TMB catalyzed by HRP, a dual-mode detection system for thrombin was established to ensure the accuracy of the test results. The method had a minimum resolution of 10 nM to the naked eye and an electrochemical detection limit as low as 0.35 nM. In addition, the sensor provided good anti-interference ability in a complex matrix and showed great potential to detect TB in complex samples.

An Insight into Nanomedicinal Approaches to Combat Viral Zoonoses

BACKGROUND

Emerging viral zoonotic diseases are one of the major obstacles to secure the "One Health" concept under the current scenario. Current prophylactic, diagnostic and therapeutic approaches often associated with certain limitations and thus proved to be insufficient for customizing rapid and efficient combating strategy against the highly transmissible pathogenic infectious agents leading to the disastrous socio-economic outcome. Moreover, most of the viral zoonoses originate from the wildlife and poor knowledge about the global virome database renders it difficult to predict future outbreaks. Thus, alternative management strategy in terms of improved prophylactic vaccines and their delivery systems; rapid and efficient diagnostics and effective targeted therapeutics are the need of the hour.

METHODS

Structured literature search has been performed with specific keywords in bibliographic databases for the accumulation of information regarding current nanomedicine interventions along with standard books for basic virology inputs.

RESULTS

Multi-arrayed applications of nanomedicine have proved to be an effective alternative in all the aspects regarding the prevention, diagnosis, and control of zoonotic viral diseases. The current review is focused to outline the applications of nanomaterials as anti-viral vaccines or vaccine/drug delivery systems, diagnostics and directly acting therapeutic agents in combating the important zoonotic viral diseases in the recent scenario along with their potential benefits, challenges and prospects to design successful control strategies.

CONCLUSION

This review provides significant introspection towards the multi-arrayed applications of nanomedicine to combat several important zoonotic viral diseases.

Recent advances in visual detection for cancer biomarkers and infectious pathogens

It is an urgency to detect infectious pathogens or cancer biomarkers using rapid, simple, convenient and cost-effective methods in complex biological samples. Many existing approaches (traditional virus culture, ELISA or PCR) for the pathogen and biomarker assays face several challenges in the clinical applications that require lengthy time, sophisticated sample pre-treatment and expensive instruments. Due to the simple and rapid detection manner as well as no requirement of expensive equipment, many visual detection methods have been considered to resolve the aforementioned problems. Meanwhile, various new materials and colorimetric/fluorescent methods have been tried to construct new biosensors for infectious pathogens and biomarkers. However, the recent progress of these aspects is rarely reviewed, especially in terms of integration of new materials, microdevice and detection mechanism into the visual detection systems. Herein, we provide a broad field of view to discuss the recent progress in the visual detection of infectious pathogens and cancer biomarkers along with the detection mechanism, new materials, novel detection methods, special targets as well as multi-functional microdevices and systems. The novel visual approaches for the infectious pathogens and biomarkers, such as bioluminescence resonance energy transfer (BRET), metal-induced metallization and clustered regularly interspaced short palindromic repeats (CRISPR)-based biosensors, are discussed. Additionally, recent advancements in visual assays utilizing various new materials for proteins, nucleic acids, viruses, exosomes and small molecules are comprehensively reviewed. Future perspectives on the visual sensing systems for infectious pathogens and cancers are also proposed.

Journey of ZnO quantum dots from undoped to rare-earth and transition metal-doped and their applications

Currently, developments in the field of quantum dots (QDs) have attracted researchers worldwide. A large variety of QDs have been discovered in the few years, which have excellent optoelectronic, antibacterial, magnetic, and other properties. However, ZnO is the single known material that can exist in the quantum state and can hold all the above properties. There is a lot of work going on in this field and we will be shorthanded if we do not accommodate this treasure at one place. This manuscript will prove to be a milestone in this noble cause. Having a tremendous potential, there is a developing enthusiasm toward the application of ZnO QDs in diverse areas. Sol-gel method being the simplest is the widely-favored synthetic method. Synthesis via this method is largely affected by a number of factors such as the reaction temperature, duration of the reaction, type of solvent, pH of the solution, and the precipitating agent. Doping enhances the optical, magnetic, anti-bacterial, anti-microbial, and other properties of ZnO QDs. However, doping elements reside mostly on the surface of the QDs. The presence of doping elements inside the core is still a major challenge for doping techniques. In this review article, we have focused on pure, rare-earth, and transition metal-doped ZnO QD properties, and the various synthetic processes and applications. Quantum confinement effect is present in nearly every aspect of the QDs. The effect of quantum confinement has also been summarized in this manuscript. Furthermore, the doping of rare earth elements and transition metal, synthetic methods for different organic molecule-capped ZnO QDs, mechanisms for reactive oxygen species (ROS) generation, drug delivery system for cancer treatment, and many more application are discussed in this paper.

Recent progress in nanomaterial-based sensing of airborne viral and bacterial pathogens

Airborne pathogens are small microbes that can cause a multitude of diseases (e.g., the common cold, flu, asthma, anthrax, tuberculosis, botulism, and pneumonia). As pathogens are transmitted from infected hosts via a number of routes (e.g., aerosolization, sneezing, and coughing), there is a great demand to accurately monitor their presence and behavior. Despite such need, conventional detection methods (e.g., colony counting, immunoassays, and various molecular techniques) generally suffer from a number of demerits (e.g., complex, time-consuming, and labor-intensive nature). To help overcome such limitations, nanomaterial-based biosensors have evolved as alternative candidates to realize portable, rapid, facile, and direct on-site identification of target microbes. In this review, nano-biosensors developed for the detection of airborne pathogens are listed and discussed in reference to conventional options. The prospects for the development of advanced nano-biosensors with enhanced accuracy and portability are also discussed.

SERS imaging-based aptasensor for ultrasensitive and reproducible detection of influenza virus A

Surface-enhanced Raman scattering (SERS)-based aptasensors display high sensitivity for influenza A/H1N1 virus detection but improved signal reproducibility is required. Therefore, in this study, we fabricated a three-dimensional (3D) nano-popcorn plasmonic substrate using the surface energy difference between a perfluorodecanethiol (PFDT) spacer and the Au layer. This energy difference led to Au nanoparticle self-assembly; neighboring nanoparticles then created multiple hotspots on the substrate. The localized surface plasmon effects at the hot spots dramatically enhanced the incident field. Quantitative evaluation of A/H1N1 virus was achieved using the decrease of Raman peak intensity resulting from the release of Cy3-labeled aptamer DNAs from nano-popcorn substrate surfaces via the interaction between the aptamer DNA and A/H1N1 virus. The use of a Raman imaging technique involving the fast mapping of all pixel points enabled the reproducible quantification of A/H1N1 virus on nano-popcorn substrates. Average ensemble effects obtained by averaging all randomly distributed hot spots mapped on the substrate made it possible to reliably quantify target viruses. The SERS-based imaging aptasensor platform proposed in this work overcomes the issues inherent in conventional approaches (the time-consuming and labor-intensiveness of RT-PCR and low sensitivity and quantitative analysis limits of lateral flow assay kits). Our SERS-based assay for detecting A/H1N1 virus had an estimated limit of detection of 97 PFU mL-1 (approximately three orders of magnitude more sensitive than that determined by the enzyme-linked immunosorbent assay) and the approximate assay time was estimated to be 20 min. Thus, this approach provides an ultrasensitive, reliable platform for detecting viral pathogens.

Reducing background absorbance via a double-lock strategy for detection of alkaline phosphatase and α-fetoprotein

Lowering the background signal for more sensitive analysis of determinands is as important as amplifying the target signal. The photoinduced oxidase of fluorescein has been reported, which can catalyze the oxidization of common substrates in a few minutes. As a metaphor for locks and keys, we designed double locks confining the activity of fluorescein to reduce the background absorbance during colorimetric detection. The first lock inhibits the main activity of fluorescein by phosphating. The second lock almost completely deactivates fluorescein by forming coordination nanoparticles (CNPs) via the self-assembly of cerium chloride and fluorescein diphosphate (FDP). The Ce-FDP CNPs are characterized by scanning electron microscope (SEM), dynamic light scattering (DLS), Fourier transform infrared spectrometer (FTIR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and energy dispersive spectrum (EDS), which show electrostatic formation and amorphous character in the morphology. Alkaline phosphatase (ALP), the key to release fluorescein, can destroy Ce-FDP CNPs along with decomposing FDP by degrading phosphate groups. Therefore, a novel colorimetric strategy for sensitive detection of ALP is established. The detection of α-fetoprotein (AFP) is further succeeded by labeling AFP antibody with ALP. By dramatically reducing the background absorbance, the detection limits of ALP and AFP are as low as 0.014 mU/mL and 0.023 ng/mL, respectively. This convenient, brief, sensitive assay provides a promising prospect for clinical diagnosis. Graphical abstract.

Pomegranate-Inspired Silica Nanotags Enable Sensitive Dual-Modal Detection of Rabies Virus Nucleoprotein

The outbreak of rabies virus (RABV) in Asia and Africa has attracted widespread concern due to its 100% mortality rate, and RABV detection is crucial to its diagnosis and treatment. Herein, we report a sensitive and reliable strategy for the dual-modal RABV detection using pomegranate-shaped dendritic silica nanospheres fabricated with densely incorporated quantum dots (QDs) and horseradish peroxidase (HRP)-labeled antibody. The immunoassay involves the specific interaction between virus and nanospheres-conjugated antibody coupled with robust fluorescence signal originating from QDs and naked-eye discernible colorimetric signal on the oxTMB. The ultrahigh loading capacity of QDs enables the detection limit down to 8 pg/mL via fluorescence modality, a 348-fold improvement as compared with conventional enzyme-linked immunosorbent assay (ELISA). In addition, the detection range was from 1.20 × 10² to 2.34 × 10⁴ pg/mL by plotting the absorbance at 652 nm with RABV concentrations with a detection limit of 91 pg/mL, which is nearly 2 order of magnitude lower than that of the conventional ELISA. Validated with 12 brain tissue samples, our immunoassay results are completely consistent with polymerase chain reaction (PCR) results. Compared with the PCR assay, our approach requires no complex sample pretreatments or expensive instruments. This is the first report on RABV diagnosis using nanomaterials for colorimetry-based prescreening and fluorescence-based quantitative detection, which may pave the way for virus-related disease diagnosis and clinical analysis.

Ultrasensitive Visualization of Virus via Explosive Catalysis of an Enzyme Muster Triggering Gold Nano-aggregate Disassembly

Sensitive and accurate diagnosis of viral infection is important for human health and social safety. Herein, by means of explosive catalysis from an enzyme muster, a powerful naked-eye readout platform has been successfully constructed for ultrasensitive immunoassay of viral entities. Liposomes were used to encapsulate multiple enzymes into an active unit. In addition, its triggered rupture could boost the disassembly of gold nano-aggregates that were cross-linked by peptides with opposite charges. As a result, plasmonically colorimetric signals were rapidly generated for naked-eye observation. Further harnessing the immunocapture, enterovirus 71 (EV71), a class of highly infective virus, was sensitively assayed with a detection limit down to 16 copies/μL. It is superior to the single enzyme-anchored immunoassay system. Most importantly, the colorimetric assay was demonstrated with 100% clinical accuracy, displaying strong anti-interference capability. It is expectable that this sensitive, accurate, and convenient strategy could provide a prospective alternative for viral infection analysis, especially in resource-constrained settings.

Multicolor colorimetric detection of ochratoxin A via structure-switching aptamer and enzyme-induced metallization of gold nanorods

Colorimetric aptasensors have been intensively studied for the ochratoxin A (OTA) detection, but they mostly exhibit just one-color change, resulting in poor visual resolution and limited use for semi-quantitative analysis. Thus, we designed a high-resolution colorimetric assay on the basis of aptamer structural switching and enzyme-induced metallization of gold nanorods (AuNRs). DNA-alkaline phosphatase (ALP)-immobilized magnetic beads were prepared. The aptamer bounded to OTA to form G-quadruplexes, releasing ALP-labelled complementary DNA (cDNA-ALP). After magnetic separation, cDNA-ALP catalyzed the decomposition of ascorbic acid 2-phosphate to ascorbic acid that reduced Ag⁺, forming an Ag shell on the surface of AuNRs. This caused a blue-shift of the longitudinal local surface plasmon resonance peak of the AuNRs and a naked eye visible multicolor change. Under optimal conditions, the assay exhibited a 9.0 nM detection limit for OTA, with high specificity. This method is promising for the on-site visual semi-quantitative detection of mycotoxins in foods.

Redox-modulated colorimetric detection of ascorbic acid and alkaline phosphatase activity with gold nanoparticles

Gold nanoparticles (AuNPs) exhibit characteristic absorption peaks in the ultraviolet visible region due to their special surface plasmon resonance effect. This characteristic absorption peak would change with the relative colour varying from wine red to orange-yellow upon sequential addition of ascorbic acid (AA) into the mixture of AuNPs and Ag(I). Similar observations also could be found when the hydrolysis product of sodium l-ascorbyl-2-phosphate with alkaline phosphatase (ALP) was used as an alternative to AA. Results of structure characterization confirmed that the phenomena were due to the reduction of Ag(I) to Ag(0) on the surface of AuNPs and the formation of core-shell AuNPs@Ag. Therefore, a colorimetric assay for rapid visual detection of AA and ALP based on redox-modulated silver deposition on AuNPs has been proposed. Under the optimal experimental conditions, the absorbance variation ΔA_{522 nm} /A_{370 nm} of AuNPs was proportional to the concentration of AA (5-60 μmol/L) and ALP (3-18 U/L) with the corresponding detection limit of 2.44 μmol/L for AA and 0.52 U/L for ALP. The assay showed excellent selectivity towards AA and ALP. Moreover, the assay has been applied to detect AA and ALP activity in real samples with satisfying results.

A simple thermometer-based photothermometric assay for alkaline phosphatase activity based on target-induced nanoprobe generation

Alkaline phosphatase (ALP)-induced in situ generation of Prussian blue nanoparticles for photothermometric ALP detection.

Smartphone-Based Point-of-Care Microfluidic Platform Fabricated with a ZnO Nanorod Template for Colorimetric Virus Detection

Viruses pose serious infectious disease threats to humans and animals. To significantly decrease the mortality and morbidity caused by virus infections, there is an urgent need of sensitive and rapid point-of-care platforms for virus detection, especially in low-resource settings. Herein, we developed a smartphone-based point-of-care platform for highly sensitive and selective detection of the avian influenza virus based on nanomaterial-enabled colorimetric detection. The 3D nanostructures, which serve as a scaffold for antibody conjugation to capture the avian influenza virus, are made on PDMS herringbone structures with a ZnO nanorod template. After virus capture, the on-chip gold nanoparticle-based colorimetric reaction allows virus detection by naked eyes with a detection limit of 2.7 × 10⁴ EID₅₀/mL, which is one order of magnitude better than that of conventional fluorescence-based ELISA. Furthermore, a smartphone imaging system with data processing capability further improves the detection limit, reaching down to 8 × 10³ EID₅₀/mL. The entire virus capture and detection process can be completed in 1.5 h. We envision that this point-of-care microfluidic system integrated with smartphone imaging and colorimetric detection would provide a fast, cheap, sensitive, and user-friendly platform for virus detection in low-resource settings.

Fluorescent-Magnetic-Catalytic Nanospheres for Dual-Modality Detection of H9N2 Avian Influenza Virus

The outbreak of H9N2 avian influenza virus (H9N2 AIV) brings high mortality and huge economic losses every year. Sensitive and reliable detection methods are essential to timely diagnosis and treatment. Herein, a dual-modality immunoassay is proposed for H9N2 AIV detection by employing fluorescent-magnetic-catalytic nanospheres (FMCNs) as labels and alkaline phosphatase (ALP)-induced metallization as a signal amplification strategy. The excellent magnetic properties of FMCNs endow the assay a potential application in complex samples. Also, the excellent fluorescence properties of FMCNs enable fluorescence modality readout. The antibodies on the FMCN surface can achieve efficient capture and separation of targets. Amplified electrochemical modality readout can be obtained through ALP-catalyzed silver deposition. Dual-modality immunoassay combined the advantages of electrochemical assay with fluorescence assay and provides accurate detection results to meet different testing needs. With two quantitative analysis forms, H9N2 AIV can be detected by electrochemical signals with a quantitation range of 0.1 to 1000 ng/mL and a detection limit of 10 pg/mL. The linear range is 300 to 1000 ng/mL with a detection limit of 69.8 ng/mL by the fluorescence signal readout. Moreover, the specificity, anti-interference ability, accuracy, and diversity of the proposal have unlimited potential for early diagnosis of suspect infections.

Colorimetric biosensors for point-of-care virus detections

Colorimetric biosensors can be used to detect a particular analyte through color changes easily by naked eyes or simple portable optical detectors for quantitative measurement. Thus, it is highly attractive for point-of-care detections of harmful viruses to prevent potential pandemic outbreak, as antiviral medication must be administered in a timely fashion. This review paper summaries existing and emerging techniques that can be employed to detect viruses through colorimetric assay design with detailed discussion of their sensing principles, performances as well as pros and cons, with an aim to provide guideline on the selection of suitable colorimetric biosensors for detecting different species of viruses. Among the colorimetric methods for virus detections, loop-mediated isothermal amplification (LAMP) method is more favourable for its faster detection, high efficiency, cheaper cost, and more reliable with high reproducible assay results. Nanoparticle-based colorimetric biosensors, on the other hand, are most suitable to be fabricated into lateral flow or lab-on-a-chip devices, and can be coupled with LAMP or portable PCR systems for highly sensitive on-site detection of viruses, which is very critical for early diagnosis of virus infections and to prevent outbreak in a swift and controlled manner.

Nanoparticle-Based LDI-MS Immunoassay for the Multiple Diagnosis of Viral Infections

Many serious public health emergencies around the globe are caused by viral epidemics. Thus, developing a reliable method for viral screening is in high demand. Multiplex assays for simultaneous detection and fast screening of high-risk pathogens are especially needed. This study employs metal nanoparticles to generate specific mass spectral signals for different RNA viruses, which enables simultaneous detection of whole viruses by laser desorption/ionization mass spectrometry (LDI-MS). We developed a nanoparticle-based sandwich immunosorbent assay as a sensing platform for the detection of viruses and viral nonstructural protein by LDI-MS. Cellulose acetate membrane (CAM) serves as the substrate for the fabrication of the sandwich immunosorbent assay with the advantages of clean mass spectra and high enrichment of analytes. Antibody-modified metal nanoparticles (Ab-MNPs; M = Au or Ag) act as metallic biocodes for the LDI-MS detection. The signal amplification readout for the virus is through the pulsed laser-induced formation of metal cluster ions ([M _n]⁺; n = 1-3) from the Ab-MNPs which specifically bind on the CAM. Our sensing system is effective for the detection of intact viruses [Enterovirus 71 (EV71) and Japanese encephalitis virus (JEV)], nonstructural protein 1 (NS1) of Zika virus (ZIKV), EV71-spiked human serum samples, and the simultaneous detection of EV71 and ZIKV. Our probe efficiently detects EV71 in real clinical serum samples with >95% agreement with RT-qPCR results. This high-throughput LDI-MS viral detection system is simple, reliable, and high-throughput. We believe this platform has the potential to be employed for the routine screening of patients with viral infections.

Gold Nanoparticle-Based Colorimetric Strategies for Chemical and Biological Sensing Applications

Gold nanoparticles are popularly used in biological and chemical sensors and their applications owing to their fascinating chemical, optical, and catalytic properties. Particularly, the use of gold nanoparticles is widespread in colorimetric assays because of their simple, cost-effective fabrication, and ease of use. More importantly, the gold nanoparticle sensor response is a visual change in color, which allows easy interpretation of results. Therefore, many studies of gold nanoparticle-based colorimetric methods have been reported, and some review articles published over the past years. Most reviews focus exclusively on a single gold nanoparticle-based colorimetric technique for one analyte of interest. In this review, we focus on the current developments in different colorimetric assay designs for the sensing of various chemical and biological samples. We summarize and classify the sensing strategies and mechanism analyses of gold nanoparticle-based detection. Additionally, typical examples of recently developed gold nanoparticle-based colorimetric methods and their applications in the detection of various analytes are presented and discussed comprehensively.

Nanomaterials-Based Colorimetric Immunoassays

Colorimetric immunoassays for tumor marker detection have attracted considerable attention due to their simplicity and high efficiency. With the achievements of nanotechnology and nanoscience, nanomaterials-based colorimetric immunoassays have been demonstrated to be promising alternatives to conventional colorimetric enzyme-linked immunoassays. This review is focused on the progress in colorimetric immunoassays with the signal amplification of nanomaterials, including nanomaterials-based artificial enzymes to catalyze the chromogenic reactions, analyte-induced aggregation or size/morphology change of nanomaterials, nanomaterials as the carriers for loading enzyme labels, and chromogenic reactions induced by the constituent elements released from nanomaterials.

Digital Single Virus Immunoassay for Ultrasensitive Multiplex Avian Influenza Virus Detection Based on Fluorescent Magnetic Multifunctional Nanospheres

The fluorescence method has made great progress in the construction of sensitive sensors but the background fluorescence of the matrix and photobleaching limit its broad application in clinical diagnosis. Here, we propose a digital single virus immunoassay for multiplex virus detection by using fluorescent magnetic multifunctional nanospheres as both capture carriers and signal labels. The superparamagnetism and strong magnetic response ability of nanospheres can realize efficient capture and separation of targets without sample pretreatment. Due to their distinguishable fluorescence imaging and photostability, the nanospheres enable single-particle counting for ultrasensitive multiplexed detection. Furthermore, the integration of digital analysis provided a reliable quantitative strategy for the detection of rare targets. Based on multifunctional nanospheres and digital analysis, a digital single virus immunoassay was proposed for simultaneous detection of H9N2, H1N1, and H7N9 avian influenza virus without complex signal amplification, whose detection limits were 0.02 pg/mL. Owing to its good specificity and anti-interference ability, the method showed great potential in single biomolecules, multiplexed detection, and early diagnosis of diseases.

Urease-induced metallization of gold nanorods for the sensitive detection of Salmonella enterica Choleraesuis through colorimetric ELISA

We applied urease-induced silver metallization on the surface of gold nanorods (AuNR) to improve colorimetric ELISA for the rapid and sensitive detection of Salmonella enterica Choleraesuis. To this end, we introduced a biotin-streptavidin system as a bridge to determine the correlation between urease and S. enterica Choleraesuis concentrations. The captured urease can catalyze the hydrolysis of urea into carbon dioxide and ammonia, and the generated ammonia can then induce the deposition of silver shell on the surface of AuNR in the presence of silver nitrate and glucose. With the decreased aspect ratio (length divided by width) of AuNR, a multicolor change of AuNR solution and a significant blue shift in the longitudinal localized surface plasmon resonance absorption peak (Δλ_max) of AuNR were obtained at the target concentrations of 1.21 × 10¹ to 1.21 × 10⁸ cfu/mL. Consequently, the detection limits of our proposed colorimetric ELISA were as low as 1.21 × 10² cfu/mL for qualitative detection with naked eyes, and 1.21 × 10¹ cfu/mL for quantitative detection, in which changes in Δλ_max of AuNR were recorded with a microplate reader. These values were at least 2 to 3 orders of magnitude lower than those obtained with conventional horseradish peroxidase-based ELISA. The analytical performance of our developed colorimetric ELISA in terms of selectivity, accuracy, reliability, and practicability were investigated by analyzing S. enterica Choleraesuis-spiked pasteurized whole milk samples.

Lighting up alkaline phosphatase in drug-induced liver injury using a new chemiluminescence resonance energy transfer nanoprobe

We present a new nanoprobe based on chemiluminescence resonance energy transfer for in vivo imaging of drug-induced alkaline phosphatase (ALP). This probe exhibits the perfect degrees of high sensitivity and specificity for real-time monitoring of ALP levels to directly evaluate drug-induced liver injury.

Ultrasensitive Virion Immunoassay Platform with Dual-Modality Based on a Multifunctional Aggregation-Induced Emission Luminogen

Sensitive and accurate detection of highly contagious virus is urgently demanded for disease diagnosis and treatment. Herein, based on a multifunctional aggregation-induced emission luminogen (AIEgen), a dual-modality readout immunoassay platform for ultrasensitive detection of viruses has been successfully demonstrated. The platform is relied on virions immuno-bridged enzymatic hydrolysis of AIEgen, accompanying with the in situ formation of highly emissive AIE aggregates and shelling of silver on gold nanoparticles. As a result, robust turn-on fluorescence and naked-eye discernible plasmonic colorimetry composed dual-signal is achieved. By further taking advantage of effective immunomagnetic enrichment, EV71 virions, as an example, can be specifically detected with a limit of detection down to 1.4 copies/μL under fluorescence modality. Additionally, semiquantitative discerning of EV71 virions is realized in a broad range from 1.3 × 10³ to 2.5 × 10⁶ copies/μL with the naked eye. Most importantly, EV71 virions in 24 real clinical samples are successfully diagnosed with 100% accuracy. Comparing to the gold standard polymerase chain reaction (PCR) assay, our immunoassay platform do not need complicated sample pretreatment and expensive instruments. This dual-modality strategy builds a good capability for both colorimetry based convenient preliminary screening and fluorescence based accurate diagnosis of suspect infections in virus-stricken areas.