FRET-Created Traffic Light Immunoassay Based on Polymer Dots for PSA Detection

Trimodal Multiplexed Lateral Flow Test Strips Assisted with a Portable Microfluidic Centrifugation Device

During the COVID-19 pandemic, the use of lateral flow assays (LFAs) expanded significantly, offering testing beyond traditional health care. Their appeal lies in the ease of use, affordability, and quick results. However, LFAs often have lower sensitivity and specificity compared with ELISA and PCR tests. Efforts to improve LFAs have increased detection times and complexity, limiting their use in large-scale point-of-care settings. To address this, we propose a novel approach using probes that generate multiple signals to enhance the sensitivity and selectivity. This concept also allows multiplexed LFAs to detect multiple analytes concurrently. We developed a trimodal probe that integrates fluorescence, color, and magnetism into a single nanohybrid. The strong plasmonic absorption and high fluorescence of Au nanoparticles and polymer dots enable qualitative and semiquantitative diagnosis, while the magnetic signal facilitates accurate quantitative measurements. As proof-of-concept targets, we selected CYFRA 21-1 and CA15-3, biomarkers for lung and breast cancer, respectively. This trimodal LFA demonstrated a remarkable detection limit of 0.26 ng/mL for CYFRA 21-1 and 2.8 U/mL for CA15-3. To the best of our knowledge, this is the first platform of a trimodal LFA with multiplexing ability. The platform's accuracy and reliability were validated using clinical serum samples, showing excellent consistency with electrochemiluminescence immunoassay results. This universal concept can be applied to other targets, paving the way for the next-generation LFAs.

Single-molecule microfluidic assay for prostate-specific antigen based on magnetic beads and upconversion nanoparticles

Early-stage diagnosis of prostatic carcinoma is essential for successful treatment and, thus, significant prognosis improvement. In laboratory practice, the standard non-invasive diagnostic approach is the immunochemical detection of the associated biomarker, prostate-specific antigen (PSA). Ultrasensitive detection of PSA is essential for both diagnostic and recurrence monitoring purposes. To achieve exceptional sensitivity, we have developed a microfluidic device with a flow-through cell for single-molecule analysis using photon-upconversion nanoparticles (UCNPs) as a detection label. For this purpose, magnetic microparticles (MBs) were first optimized for the capture and preconcentration of PSA and then used to implement a bead-based upconversion-linked immunoassay (ULISA) in the microfluidic device. The digital readout based on counting single nanoparticle-labeled PSA molecules on MBs enabled a detection limit of 1.04 pg mL-1 (36 fM) in 50% fetal bovine serum, which is an 11-fold improvement over the respective analog MB-based ULISA. The microfluidic technique conferred several other advantages, such as easy implementation and the potential for achieving high-throughput analysis. Finally, it was proven that the microfluidic setup is suitable for clinical sample analysis, showing a good correlation with a reference electrochemiluminescence assay (recovery rates between 97% and 105%).

Nanoenzyme Hydrogel Film-Based Portable Point-of-Care Testing Platform for Double-Signal Visual Detection of PSA

The development of the Point-of-Care Testing (POCT) platform that combines convenience and cost-effectiveness is crucial for enabling the visual detection of disease biomarkers. In this work, a POCT platform for the sensitive in situ detection of prostate specific antigen (PSA) with dual-signal output was constructed by functionalizing the Eppendorf (EP) tube. This was achieved through the modification of aptamer hairpin probes (AHPs) on the lid of the EP tube and the assembly of a nanoenzyme hydrogel film on its inner wall. The target could trigger the release of Ag+ by AHP and subsequently activate Ag+-dependent DNAzyme (Ag-DNAzyme). This would initiate the cleavage of the DNA-Au/Pt NP hydrogel network, leading to the release of Au/Pt NPs. The released Au/Pt NPs exhibit both peroxidase (POD)-like and catalase (CAT)-like activity to produce a colorimetric response and induce liquid flow under pressure. Therefore, the target can be measured visually and quantitatively through colorimetric analysis and the measurement of total dissolved solids (TDS) using a pressure-triggered liquid flow device integrated into the platform. The designed platform is distinguished by its simplicity, specificity, cost-effectiveness, and remarkable sensitivity. It allows for the visual detection of PSA within concentration ranges of 0.5-100 ng/L (colorimetric) and 3-100 ng/L (TDS reading), boasting detection limits as low as 0.15 ng/L (colorimetric) and 0.57 ng/L (TDS reading). The strategy of target-triggered nanoenzyme release significantly enhances sensitivity and provides a guiding approach for visual biomarker detection.

Semiconducting polymer dots for multifunctional integrated nanomedicine carriers

The expansion applications of semiconducting polymer dots (Pdots) among optical nanomaterial field have long posed a challenge for researchers, promoting their intelligent application in multifunctional nano-imaging systems and integrated nanomedicine carriers for diagnosis and treatment. Despite notable progress, several inadequacies still persist in the field of Pdots, including the development of simplified near-infrared (NIR) optical nanoprobes, elucidation of their inherent biological behavior, and integration of information processing and nanotechnology into biomedical applications. This review aims to comprehensively elucidate the current status of Pdots as a classical nanophotonic material by discussing its advantages and limitations in terms of biocompatibility, adaptability to microenvironments in vivo, etc. Multifunctional integration and surface chemistry play crucial roles in realizing the intelligent application of Pdots. Information visualization based on their optical and physicochemical properties is pivotal for achieving detection, sensing, and labeling probes. Therefore, we have refined the underlying mechanisms and constructed multiple comprehensive original mechanism summaries to establish a benchmark. Additionally, we have explored the cross-linking interactions between Pdots and nanomedicine, potential yet complete biological metabolic pathways, future research directions, and innovative solutions for integrating diagnosis and treatment strategies. This review presents the possible expectations and valuable insights for advancing Pdots, specifically from chemical, medical, and photophysical practitioners' standpoints.

Nanoparticle-Based Bioaffinity Assays: From the Research Laboratory to the Market

Advances in the development of new biorecognition elements, nanoparticle-based labels as well as instrumentation have inspired the design of new bioaffinity assays. This review critically discusses the potential of nanoparticles to replace current enzymatic or molecular labels in immunoassays and other bioaffinity assays. Successful implementations of nanoparticles in commercial assays and the need for rapid tests incorporating nanoparticles in different roles such as capture support, signal generation elements, and signal amplification systems are highlighted. The limited number of nanoparticles applied in current commercial assays can be explained by challenges associated with the analysis of real samples (e.g., blood, urine, or nasal swabs) that are difficult to resolve, particularly if the same performance can be achieved more easily by conventional labels. Lateral flow assays that are based on the visual detection of the red-colored line formed by colloidal gold are a notable exception, exemplified by SARS-CoV-2 rapid antigen tests that have moved from initial laboratory testing to widespread market adaption in less than two years.

Point-of-Care Prostate Specific Antigen Testing: Examining Translational Progress toward Clinical Implementation

Prostate cancer (PCa) is the second most common male cancer and is attributable to over 375,000 deaths annually. Prostate specific antigen (PSA) is a key biomarker for PCa and therefore measuring patient PSA levels is an important aspect of the diagnostic pathway. Automated immunoassays are currently utilized for PSA analysis, but they require a laboratory setting with specialized equipment and trained personnel. This results in high diagnostic costs, extended therapeutic turnaround times, and restrictions on testing capabilities in resource-limited settings. Consequently, there is a strong drive to develop point-of-care (PoC) PSA tests that can offer accurate, low-cost, and rapid results at the time and place of the patient. However, many emerging PoC tests experience a trade-off between accuracy, affordability, and accessibility which distinctly limits their translational potential. This review comprehensively assesses the translational advantages and limitations of emerging laboratory-level and commercial PoC tests for PSA determination. Electrochemical and optical PSA sensors from 2013 to 2023 are systematically examined. Furthermore, we suggest how the translational potential of emerging tests can be optimized to achieve clinical implementation and thus improve PCa diagnosis globally.

Hybrid polymer dot-magnetic nanoparticle based immunoassay for dual-mode multiplexed detection of two mycotoxins

We designed polymer dot-magnetic nanoparticle nanohybrids for signal enhancement in a test strip platform. Besides, the multicolor emissions of the Pdots embed multiplexing ability for this test strip. Two mycotoxins, aflatoxin B1 and zearalenone, were tested with the determined limits of detection of 2.15 ng mL-1 and 4.87 ng mL-1, respectively.

Fabrication of Biomimetic Cascade Nanoreactor Based on Covalent Organic Framework Capsule for Biosensing

The cooperation of biocatalysis and chemocatalysis in a catalytic cascade reaction has received extensive attention in recent years, whereas its practical applications are still hampered due to the fragility of the enzymes, poor compatibility between the carriers and enzymes, and limited catalytic efficiency. Herein, a biomimetic cascade nanoreactor (GOx@COFs@Os) was presented by integrating glucose oxidase (GOx) and Os nanozyme with covalent organic framework (COF) capsule using metal-organic framework (ZIF-90) as a template. The obtained GOx@COFs@Os capsule provided a capacious microenvironment to retain the conformational freedom of GOx for maintaining its activity, wherein the enzyme activity of GOx in COF capsules was equal to 92.9% of the free enzyme and was 1.88-folds higher than that encapsulated in ZIF-90. Meanwhile, the COF capsule could protect the GOx against incompatible environments (high temperature, acid, and organic solvents), resulting in improved stability of the packaged enzymes. Moreover, the COF capsule with great pore structure significantly improved the affinity to substrates and facilitated efficient mass transfer, which achieved 2.19-folds improvement in catalytic efficiency than the free cascade system, displaying the great catalytic performance in the cascade reaction. More importantly, the biomimetic cascade capsule was successfully employed for glucose monitoring, glutathione sensing, and bisphenol S detection in the immunoassay as a proof-of-concept. Our strategy provided a new avenue in the improvement of biocatalytic cascade performance to encourage its wide applications in various fields.

N/S-Co-doped carbon dot-based FRET ratiometric fluorescence aptasensing platform modulated with entropy-driven DNA amplifier for ochratoxin A detection

This study proposes a nitrogen and sulfur co-doped carbon dot (N/S-CD)-based FRET ratiometric fluorescence aptasensing strategy modulated with entropy-driven DNA amplifier for sensitive and accurate detection of ochratoxin A (OTA). In the strategy, a duplex DNA probe containing OTA aptamer and complementary DNA (cDNA) is designed as a recognition and transformation element. Upon sensing of target OTA, the cDNA was liberated, and triggered a three-chain DNA composite-based entropy-driven DNA circuit amplification, making CuO probes anchor on a magnetic bead (MB). The CuO-encoded MB complex probe is finally turned into abundant Cu²⁺, which oxidizes o-phenylenediamine (oPD) to generate 2,3-diaminophenazine (DAP) with yellow fluorescence and further triggers FRET between the blue fluorescent N/S-CDs and DAP. The changes in ratiometric fluorescence are related to the OTA concentration. Originating from the synergistic amplifications from the entropy-driven DNA circuits and Cu²⁺ amplification, the strategy dramatically enhanced detection performance. A limit of detection as low as 0.006 pg/mL of OTA was achieved. Significantly, the aptasensor can visually evaluate the OTA via on-site visual screening. Moreover, the high-confidence quantification of the OTA in real samples with results consistent with that of the LC-MS method indicated that the proposed strategy has practical application prospects for sensitive and accurate quantification in food safety.

Tuning iron spin states in single-atom nanozymes enables efficient peroxidase mimicking

The large-scale application of nanozymes remains a significant challenge owing to their unsatisfactory catalytic performances. Featuring a unique electronic structure and coordination environment, single-atom nanozymes provide great opportunities to vividly mimic the specific metal catalytic center of natural enzymes and achieve superior enzyme-like activity. In this study, the spin state engineering of Fe single-atom nanozymes (FeNC) is employed to enhance their peroxidase-like activity. Pd nanoclusters (PdNC) are introduced into FeNC, whose electron-withdrawing properties rearrange the spin electron occupation in Fe(ii) of FeNC-PdNC from low spin to medium spin, facilitating the heterolysis of H2O2 and timely desorption of H2O. The spin-rearranged FeNC-PdNC exhibits greater H2O2 activation activity and rapid reaction kinetics compared to those of FeNC. As a proof of concept, FeNC-PdNC is used in the immunosorbent assay for the colorimetric detection of prostate-specific antigen and achieves an ultralow detection limit of 0.38 pg mL-1. Our spin-state engineering strategy provides a fundamental understanding of the catalytic mechanism of nanozymes and facilitates the design of advanced enzyme mimics.

Semiconducting polymer dots as fluorescent probes for in vitro biosensing

Semiconducting polymer dots (Pdots) have emerged as novel fluorescent probes with excellent characteristics, such as ultrahigh molar extinction coefficient, easy tunable absorption and emission bands, high brightness, and excellent photostability. Combined with good biocompatibility properties, much effort has been devoted to Pdots for in vivo biological imaging and therapy applications, such as deep-tissue fluorescent imaging, photodynamic therapy, photothermal therapy, and nanocarriers of genes or chemical drugs. Many reviews have been presented in these fields. On the other hand, a large number of studies employing Pdots for in vitro biosensing applications have been reported during the past few years, and there are barely any relevant reports to summarize the progress in this area. Hence, it is necessary to review these studies to promote the comprehensive application of Pdots. Herein, we introduce the properties and functionalization of Pdots, and systematically summarize the progress in the in vitro applications of Pdots, including the detection of DNAs, microRNAs, proteins, enzymatic activity, and some biological small molecules and ions. Finally, we share our perspectives on the future direction of this field.

Construction of Effective Nanosensor by Combining Semiconducting Polymer Dots with Diphenylcarbazide for Specific Recognition of Trace Cr (VI) Ion in Water and Vitro

Hexavalent chromium (Cr (VI)) ion, as highly toxic environmental pollution, severely endangers the ecological environment and public health. Herein, a fluorescent nanosensor (PFO-DPC) was constructed by combining semiconducting polymer dots with diphenylcarbazide (DPC) for sensing Cr (VI) ion in aqueous solution and living cells. DPC and poly (styrene-co-maleic anhydride) (PSMA) polymer mixed with polyfluorene (PFO) were utilized for selectively indicating Cr (VI) ion and improving the efficiency of detection, respectively. The presence of Cr (VI) ion effectively turned off the blue and green fluorescence of PFO-DPC in the aqueous environment, and the fluorescence quenching efficiency exhibited a good linear relationship between the range of 0.0 to 2.31 nM (R2 = 0.983) with a limit of detection (LOD) of 0.16 nM. The mechanism of fluorescence quenching could possibly be attributed to the internal filtration effect (IFE). Additionally, PFO-DPC showed a satisfactory performance in monitoring intracellular Cr (VI) ion. Our results indicate that the sensor is promising in various applications.

New Approach to Generate Ratiometric Signals on Immunochromatographic Strips for Small Molecules

The self-calibration capability of ratiometric signals has been widely considered to enhance the accuracy, sensitivity, and anti-interference ability of immunoassays. Exploring a new approach to generate ratiometric signals can provide more options for various requirements. Herein, we integrated the negative-readout competitive and positive-readout noncompetitive immunoassays into a single assay by employing different color tracers, labeled peptidomimetic and anti-immunocomplex peptides, to create a new unconstrained ratiometric signal approach. Using an immunochromatographic strip (ICS) and a fungicide benzothiostrobin as the analytical platform and analyte, respectively, we showed that this approach can be extensively applied to fluorescence and colorimetry readouts, which have also been proven for strong anti-interference ability to an external light environment. Moreover, the enormous intuitional color changes of ratiometric fluorescent and colorimetric ICSs (RFICS and RCICS) enabled the formation of the color reference cards (like the pH paper) for visual judgment. After adaptation with a portable smartphone, the quantitative detection limits for RFICS and RCICS were 0.17 and 0.44 ng mL^-1, respectively. In addition, the ICSs showed good accuracy for the detection of benzothiostrobin in spiked samples.

A Portable Biosensor Based on Au Nanoflower Interface Combined with Electrochemical Immunochromatography for POC Detection of Prostate-Specific Antigen

Serum prostate-specific antigen (PSA) is a widely used for the detection of prostate cancer and is considered the most reliable biomarker. However, the currently reported detection methods cannot achieve rapid monitoring. Here, we report a novel electrochemical immunochromatography (EIC) system for clinically accurate PSA detection. First, we constructed a carbon interface modified with gold nanoflowers (Au NFs) based on screen-printed carbon electrodes (SPCE), which acted as nanostructures with larger specific surface area that increased the number of PSA capture antibodies and can further improve detection signal-to-noise (S/N) ratio. Then, we fabricated detection chips by combining the SPCE/Au NFs with EIC. Under optimized conditions, the proposed biosensor exhibits high accuracy, taking only 15 minutes to complete detection. By measuring the levels of PSA in clinical blood samples, the biosensor can successfully discriminate clinically diagnosed prostate cancer patients from healthy controls.

Tetra-primer ARMS-PCR combined with dual-color fluorescent lateral flow assay for the discrimination of SARS-CoV-2 and its mutations with a handheld wireless reader

Several virulent variants of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) have emerged along with the spread of this virus throughout the population. Some variants can exhibit increased transmissibility and reduced immune neutralization reactivity. These changes are deeply concerning issues that may hinder the ongoing effort of epidemic control measures, especially mass vaccination campaigns. The accurate discrimination of SARS-CoV-2 and its emerging variants is essential to contain the coronavirus disease 2019 pandemic. Herein, we report a low-cost, facile, and highly sensitive diagnostic platform that can simultaneously distinguish wild-type (WT) SARS-CoV-2 and its two mutations, namely, D614G and N501Y, within 2 h. WT or mutant (M) nucleic acid fragments at each allelic locus were selectively amplified by the tetra-primer amplification refractory mutation system (ARMS)-PCR assay. Allele-specific amplicons were simultaneously detected by two test lines on a quantum dot nanobead (QB)-based dual-color fluorescent test strip, which could be interpreted by the naked eye or by a home-made fluorescent strip readout device that was wirelessly connected to a smartphone for quantitative data analysis and result presentation. The WT and M viruses were indicated and were strictly discriminated by the presence of a green or red band on test line 1 for the D614G site and test line 2 for the N501Y site. The limits of detection (LODs) for the WT and M D614G were estimated as 78.91 and 33.53 copies per μL, respectively. This assay was also modified for the simultaneous detection of the N and ORF1ab genes of SARS-CoV-2 with LODs of 1.90 and 6.07 copies per μL, respectively. The proposed platform can provide a simple, accurate, and affordable diagnostic approach for the screening of SARS-CoV-2 and its variants of concern even in resource-limited settings.

Photothermal card reader assay using the commercial colloidal gold test strip for the rapid quantitative detection of food hazards

The simple and rapid commercial colloidal gold test strip can only be used for qualitative or semi-quantitative detection, accompanied by weak detectability and false negative experimental results. Herein, a photothermal test strip assay which combined test strip with a portable photothermal card reader was established to achieve quantitative detection with excellent detectability. According to the photothermal effect produced by gold nanoparticles (GNPs) captured on the test line, the signal could be recorded by the reader. Thirteen food hazards including veterinary drug residues and pesticide residues were tested; the photothermal detectability in actual samples were about 23 (methyl parathion), 7 (enrofloxacin), 6 (sarafloxacin), 8 (sulfadiazine), 12 (sulfamethazine), 7 (paraquat), 6 (malachite green), 11 (amantadine), 13 (nitrofurazone), 6 (diethylstilbestrol), 12 (estriol), 21 (estrone), and 26 (17β-estradiol) times better than the visual detectability. Our results demonstrated that the photothermal test strip assay could be used for sensitive, rapid, and quantitative detection of residues of food hazards.

Traffic light type paper-based analytical device for intuitive and semi-quantitative naked-eye signal readout

Microfluidic paper-based analytical devices (μPADs) have attracted great attention as potential candidates for point-of-care testing (POCT). Nevertheless, only a limited number of μPADs expected to satisfy the standard of Clinical Laboratory Improvement Amendments (CLIA) waived tests as issued by the US Food and Drug Administration (FDA) have been reported. This work introduces a "traffic light type μPAD", enabling highly intuitive semi-quantitative equipment-free naked-eye readout with no need for calibration, subjective interpretation or calculation. Assay results are displayed as traffic light colours reporting 5 analyte concentration levels (green/green & yellow/yellow/yellow & red/red). The device has been designed to never display all three colours simultaneously, eliminating any risk for misinterpretation. The mechanism relies on the modulation of sample flow through a network of paperfluidic channels modified with a hydrophobic to hydrophilic phase-switching substance responsive to H₂O₂. User operation is limited to sample application, followed by observing a clear and time-independent traffic light signal after approximately 10-30 min. Multiple factors influencing the H₂O₂ concentration-dependent appearance of a specific traffic light signal were studied. Making use of the possibilities for customising the concentration threshold levels for traffic light colour appearance, quantification of glucose at 5 levels in a clinically relevant concentration range was demonstrated in artificial urine as a model proof-of-concept. This platform is expected to offer the possibility for the future detection of other important metabolites.

Crystal-reconstructed BiVO4 semiconductor photoelectrochemical sensor for ultra-sensitive tumor biomarker detection

In this study, we developed a crystal-reconstructed-BiVO₄ aptamer photoelectrochemical (PEC) biosensor by a high-energy laser treatment technique. This biosensor achieves a limit of detection (LOD) (0.82 ag mL^-1), linear detection range (1 ag mL^-1 to 2 ng mL^-1), and resolution ratio (∼18 molecules per mL) for prostate-specific antigen (PSA) tumor biomarker detection. Furthermore, reconstructed surface microstructure and oxygen vacancy doping energy formation after crystal reconstruction induce the stereo-hindrance effect and photogenerated hole energy is reduced during PSA target detection. In this case, a photocurrent inhibition phenomenon for PSA detection is noticed. Based on this photocurrent inversion phenomenon, some dysoxidizable nucleonic acid tumor (miRNA-21) and virus biomarkers (RdRp-COVID) can be detected with a LOD level of ∼10^-16 M by linking the corresponding base paring probe on the surface of the crystal-reconstructed photoanode. In addition to high sensitivity, this PEC biosensor presents high detection specificity, stability, and accuracy in clinical verification. Thus, this crystal-reconstructed PEC biosensor shows application potential in the fields of multi-tumor or viral biomarker detection.

Photoluminescent Molecules and Materials as Diagnostic Reporters in Lateral Flow Assays

The lateral flow assay (LFA) is a point-of-care diagnostic test commonly available in an over-the-counter format because of its simplicity, speed, low cost, and portability. The reporter particles in these assays are among their most significant components because they perform the diagnostic readout and dictate the test's sensitivity. Today, gold nanoparticles are frequently used as reporters, but recent work focusing on photoluminescent-based reporter technologies has pushed LFAs to better performance. These efforts have focused specifically on reporters made of organic fluorophores, quantum dots, lanthanide chelates, persistent luminescent phosphors, and upconversion phosphors. In most cases, photoluminescent reporters show enhanced sensitivity compared to conventional gold nanoparticle-based assays. Here, we examine the advantages and disadvantages of these different reporters and highlight their potential benefits in LFAs. Our assessment shows that photoluminescent-based LFAs can not only reach lower detection limits than LFAs with traditional reporters, but they also can be capable of quantitative and multiplex analyte detection. As a result, the photoluminescent reporters make LFAs well-suited for medical diagnostics, the food and agricultural industry, and environmental testing.

Bimodal Multiplexed Detection of Tumor Markers in Non-Small Cell Lung Cancer with Polymer Dot-Based Immunoassay

Semiconducting polymer nanoparticles (Pdots) have been demonstrated to be a promising class of probes for use in fluorometric immunochromatographic test strips (ICTS). The advantages of Pdots in ICTSs include ultrahigh brightness, minimal nonspecific adsorption, and multicolor availability, which together contribute to the high sensitivity, good specificity, and multiplexing ability. These unique properties can therefore circumvent several significant challenges of commercial ICTSs, including insufficient specificity/sensitivity and difficulty in quantitative and multiplexed detection. Here, we developed a colorimetric and fluorescent bimodal readout ICTS based on gold-Pdot nanohybrids for the determination of carcinoembryonic antigen (CEA) and cytokeratin 19 fragment (CYFRA 21-1) expressed abnormally in human blood of non-small-cell lung cancer (NSCLS). The vivid color from Au nanomaterials can be used for rapid qualitative screening (colorimetry) in 15 min, while the bright fluorescence of Pdots is ideal for the advanced quantitative measurements of CEA and CYFRA21-1 concentrations in whole blood samples. This bimodal ICTS platform possesses phenomenal detection sensitivity of 0.07 and 0.12 ng/mL for CYFRA21-1 and CEA, respectively. The accuracy and reliability of this ICTS platform were further evaluated with clinical serum samples from NSCLS patients at different stages, showing good consistency with the results from electrochemiluminescence immunoassay.

A Lab-in-a-Syringe Device Integrated with a Smartphone Platform: Colorimetric and Fluorescent Dual-Mode Signals for On-Site Detection of Organophosphorus Pesticides

Herein, a portable lab-in-a-syringe device integrated with a smartphone sensing platform was designed for rapid, visual quantitative determination of organophosphorus pesticides (OPs) via colorimetric and fluorescent signals. The device was chiefly made up of a conjugate pad labeled with cetyltrimethylammonium bromide-coated gold nanoparticles (CTAB-Au NPs) and a sensing pad modified by ratiometric probes (red-emission quantum dots@SiO₂ nanoparticles@green-emission quantum dots, rQDs@SiO₂@gQDs probe), which was assembled through a disposable syringe and reusable plastic filter. In the detection system, thiocholine (Tch), the hydrolysis product of thioacetylcholine (ATch) by acetylcholinesterase (AchE), could trigger the aggregation of CTAB-Au NPs, resulting in a significant color change from red to purple. Then, CTAB-Au NPs flowed vertically upward and bound to the rQDs@SiO₂@gQDs probe on the sensing pad, reducing the fluorescence resonance energy transfer effect between CTAB-Au NPs and gQDs. Meanwhile, rQDs embedded in SiO₂ NPs remained stable as internal reference fluorescence, achieving a color transition from red to green. Thus, based on the inhibition of AChE activity by OPs, a colorimetric and fluorescent dual-mode platform was constructed for on-site detection of OPs. Using glyphosate as a model, with the support of a color recognizer application (APP) on a smartphone, the ratio of red and green channel values could be utilized for accurate OP quantitative analysis ranging from 0 to 10 μM with a detection limit of 2.81 nM (recoveries, 90.8-122.4%; CV, 1.2-3.4%). Overall, the portable lab-in-a-syringe device based on a smartphone sensing platform integrated sample monitoring and result analysis in the field, implying great potential for on-site detection of OPs.

Fe-N-C Single-Atom Catalyst Coupling with Pt Clusters Boosts Peroxidase-like Activity for Cascade-Amplified Colorimetric Immunoassay

Although single-atom catalysts with high enzyme-like activities have been found, the rational design of highly active peroxidase (POD)-like nanozymes is still a formidable challenge. Herein, highly active POD-like nanozymes were synthesized through loading Pt clusters on the Fe single-atom (Fe_SA-Pt_C) nanozymes. The POD-like activity of Fe_SA-Pt_C nanozymes is enhanced 4.5-fold and 7-fold, in comparison to that of Fe_SA and Pt_C nanozymes, respectively, which is attributed to the unexpected synergistic effect between Fe single atoms and Pt clusters. Based on the outstanding POD-like activity of Fe_SA-Pt_C nanozymes, a cascade signal amplification strategy was constructed by combining glucose oxidase for the colorimetric biosensing of prostate-specific antigens, exhibiting satisfactory sensitivity, high selectivity, a low detection limit of 1.8 pg/mL, and practical feasibility in serum sample detection. This work may serve as a tough foundation to guide the design of superior POD-like nanozymes and expand the application in biosensing.

Recent advances in FRET-Based biosensors for biomedical applications

Fluorescence resonance energy transfer (FRET)-based biosensors are effective analytical tools extensively used in fields of biomedicine, pharmacology, toxicology, and food sciences. Ratiometric imaging of substantial cellular processes, molecular components, and biological interactions is widely performed by these biosensors. A variety of FRET-based biosensors have provided comprehensive insights into underlying mechanisms of pathological conditions in live cells, tissues, and organisms. Moreover, integration of FRET-based biosensors with the current bioanalytical techniques allows for accurate, rapid, and sensitive diagnosis and proposes the advanced strategies for treatment. Precise analysis of ligand-receptor interactions by FRET-based biosensors has presented a basis for determination of novel therapeutic agents. Therefore, this study was designed to review the recent developments in FRET-based biosensors and their biomedical applications. In addition, characteristics, challenges, and outlooks of these biosensors were discussed.

Signal-on photoelectrochemical immunoassay mediated by the etching reaction of oxygen/phosphorus co-doped g-C3N4/AgBr/MnO2 nanohybrids

We designed a signal-on photoelectrochemical (PEC) immunoassay for the sensitive monitoring of prostate-specific antigen (PSA) based on the etching reaction of hydrogen peroxide (H₂O₂) toward oxygen/phosphorus co-doped graphitic C₃N₄/AgBr/MnO₂ nanosheets (OP-g-C₃N₄/AgBr/MnO₂). Initially, glucose oxidase (GO_X)-labeled detection antibodies were introduced into the capture antibody-coated microplate with a sandwich-type immunoreaction in the presence of PSA. Then, the as-generated H₂O₂ from the decomposition of glucose by GO_X etched the manganese dioxide (MnO₂) nanosheets into manganese ions (Mn²⁺), thereby causing the exposure of the underlying OP-g-C₃N₄/AgBr. Meanwhile, H₂O₂ could be also used as an electron scavenger, and restrain the recombination of the electron-hole pairs of OP-g-C₃N₄/AgBr. Two advantages of H₂O₂ enhanced the photocurrent synergistically. Under optimum conditions, the PEC immunoassay showed high sensitivity toward target PSA within a dynamic working range of 0.05-50 ng mL^-1 with a limit of detection of 17 pg mL^-1. In addition, our system possessed high specificity, favorable selectivity, and good stability. Relative to commercialized PSA ELISA kits, the accuracy of our strategy was acceptable. More importantly, our strategy can be easily extended to screen other biomarkers by controlling the corresponding antibodies.

Ratiometric Fluorescent Lateral Flow Immunoassay for Point-of-Care Testing of Acute Myocardial Infarction

We report the development of a highly sensitive ratiometric fluorescent lateral flow immunoassay (RFLFIA) strip for rapid and accurate detection of acute myocardial infarction biomarker, namely heart-type fatty acid binding protein (H-FABP). The RFLFIA strip works in terms of ratiometric change of fluorescence signal, arising from blending of fluorescence emitted by two composite nanostructures conjugated to capture and probe antibodies and inner filter effect of gold nanoparticles. In conjunction with using custom smartphone-based analytical device and tonality analysis, quantitative detection of H-FABP was achieved with a low limit of detection at 0.21 ng mL^-1 . The RFLFIA strip can generate a visually distinguishable green-to-red color change around the threshold concentration of H-FABP (6.2 ng mL^-1 ), thus allowing the semi-quantitative diagnosis by the naked eye.

Diverse Dyes-Embedded Staphylococcus aureus as Potential Biocarriers for Enhancing Sensitivity in Biosensing

Nanomaterials-based immunochromatographic assays (ICAs) have gained great commercial success in real-life point-of-care testing (POCT). Exploring novel carriers of ICAs with improved signaling and sustained activity favors the development of sensitive POCT. Herein a potent signal biotag, colored Staphylococcus aureus (SA), was created for ICA carriers through a mild self-assembly strategy, providing high luminance and abundant specific binding sites for immobilization of monoclonal antibodies (mAbs). The biocompatible SA-dyes (SADs) retained both an intact surface structure for mAbs labeling (Fc portion) and the superior bioactivity of immobilized mAbs (affinity constant was about 10⁹ M^-1), thus waiving the intrinsic limitations of traditional nanomaterials and endowing high sensitivity. Proof-of-concept was demonstrated by employing Congo red- or/and fluorescein isothiocyanate-embedded SA (SACR, SAFITC, and SACR-SAFITC) as ICA carriers to detect zearalenone (ZEN) through colorimetric or/and fluorimetric signals. Furthermore, the ICAs satisfied the clinical requirement perfectly, including limit of detection (0.013 ng/mL, which was at least an 85-fold improvement over that of traditional gold nanoparticles-based ICA), linearity (R² > 0.98), reproducibility (RSD < 8%), selectivity, and stability. Importantly, the proposed biosensors could be well-applied in four real samples for ZEN monitoring with satisfactory recoveries, correlating well with the results from liquid chromatography-tandem mass spectrometry (LC-MS). This work also proved a universal design for tailoring coloration bands for SAD-ICA detection of multiple analytes.

Ultrasensitive Microfluidic Paper-Based Electrochemical/Visual Analytical Device via Signal Amplification of Pd@Hollow Zn/Co Core-Shell ZIF67/ZIF8 Nanoparticles for Prostate-Specific Antigen Detection

An effective signal amplification strategy is essential to enhance the analytical performance of microfluidic paper-based analytical devices (μPADs) for tracing biomarkers. Here, a simple but efficient approach with superior electrocatalytic performance of Pd@hollow Zn/Co core-shell ZIF67/ZIF8 nanoparticles for regulating the efficacious signal amplification process was utilized to realize the detection of prostate-specific antigen (PSA). By rationally designing the core-shell structure of ZIF67/ZIF8 with hollow characteristics on the nanoscale and introducing the noble metal element Pd into the cavity, the diffusion limitation and porous confinement reduction of the obtained nanomaterials with uniform morphology and satisfactory chemical stability could be realized, which endowed it with better catalytic performance than solid metal-organic frameworks (MOFs) and ensured effective signal amplification of H₂O₂ reduction for achieving enhanced electrochemical signals. Moreover, with the assistance of signal probes, the remaining H₂O₂ could flow to the color area to catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine to form a colored product by changing the spatial configuration of the μPAD, thus realizing the visual detection of PSA. On the basis of this novel analytical device, dual-mode ultrasensitive detection of PSA could be achieved with a lower limit of detection of 0.78 pg/mL (S/N = 3) and a wider linear range from 5 pg/mL to 50 ng/mL. This work provided the opportunity of introducing the noble metal element Pd into the cavity of the MOF hollow structure to improve its electrocatalytic efficiency and construct a high-performance μPAD for clinical detection of other biomarkers.

Recent Developments in Semiconducting Polymer Dots for Analytical Detection and NIR-II Fluorescence Imaging

In recent years, semiconducting polymer dots (Pdots) have attracted enormous attention in applications from fundamental analytical detection to advanced deep-tissue bioimaging due to their ultrahigh fluorescence brightness with excellent photostability and minimal cytotoxicity. Pdots have therefore been widely adopted for a variety types of molecular sensing for analytical detection. More importantly, the recent development of Pdots for use in the optical window between 1000 and 1700 nm, popularly known as the "second near-infrared window" (NIR-II), has emerged as a class of optical transparent imaging technology in the living body. The advantages of the NIR-II region over the traditional NIR-I (700-900 nm) window in fluorescence imaging originate from the reduced autofluorescence, minimal absorption and scattering of light, and improved penetration depths to yield high spatiotemporal images for biological tissues. Herein, we discuss and summarize the recent developments of Pdots employed for analytical detection and NIR-II fluorescence imaging. Starting with their preparation, the recent developments for targeting various analytes are then highlighted. After that, the importance of and latest progress in NIR-II fluorescence imaging using Pdots are reported. Finally, perspectives and challenges associated with the emergence of Pdots in different fields are given.

Preparation of species-specific monoclonal antibody and development of fluorescence immunoassay based on fluorescence resonance energy transfer of carbon dots for accurate and sensitive detection of Alicyclobacillus acidoterrestris in apple juice

The growth and metabolism of Alicyclobacillus acidoterrestris can lead to the spoilage of commercial fruit juice. Existing methods have some drawbacks such as complex sample pretreatment, skilled technician requirement, reduced sensitivity and specificity. Herein, a novel fluorescence immunoassay was developed using a monoclonal antibody (mAb) against A. acidoterrestris as the sensing element and carbon dots (CDs) as the signal response unit. The CDs can be quenched via fluorescence resonance energy transfer (FRET) by the oxidization product of p-phenylenediamine (PPD), a chromogenic substrate of horseradish peroxidase (HRP). This approach showed enhanced accuracy and sensitivity with relatively low limit of detection (LOD) of 6.16 × 10² CFU mL^-1. Moreover, apple juice contaminated with 1 CFU mL^-1 of A. acidoterrestris can be identified after 24 h enrichment. This fluorescence immunoassay could serve as a powerful tool for laboratory identification and on-site inspection of A. acidoterrestris, reducing the adverse effect on the quality of fruit juice.

Luminous silica colloids with carbon dot incorporation for sensitive immunochromatographic assay of Zika virus

Fluorescent SiO₂ colloids prepared from silanized CDs embedded in silica spheres were integrated with an immunochromatographic platform for Zika virus detection.

Förster resonance energy transfer (FRET) and applications thereof

FRET is a nonradiative process of energy transfer that is based on the dipole-dipole interactions between molecules that are fluorescent. Transfer of energy takes place rapidly from a donor molecule to an acceptor molecule in juxtaposition such as 0 to 10 nm without photonic radiation. FRET has occupied a center stage in biotechnology and biological studies. It is used to gain information on conformation changes in single molecules. The pharmaceutical industry has also developed large fluorescence detection systems with very small sample sizes, at the level of single molecules, using fluorescence correlation spectroscopy.

Photoluminescent and Chromic Nanomaterials for Anticounterfeiting Technologies: Recent Advances and Future Challenges

Counterfeiting and inverse engineering of security and confidential documents, such as banknotes, passports, national cards, certificates, and valuable products, has significantly been increased, which is a major challenge for governments, companies, and customers. From recent global reports published in 2017, the counterfeiting market was evaluated to be $107.26 billion in 2016 and forecasted to reach $206.57 billion by 2021 at a compound annual growth rate of 14.0%. Development of anticounterfeiting and authentication technologies with multilevel securities is a powerful solution to overcome this challenge. Stimuli-chromic (photochromic, hydrochromic, and thermochromic) and photoluminescent (fluorescent and phosphorescent) compounds are the most significant and applicable materials for development of complex anticounterfeiting inks with a high-security level and fast authentication. Highly efficient anticounterfeiting and authentication technologies have been developed to reach high security and efficiency. Applicable materials for anticounterfeiting applications are generally based on photochromic and photoluminescent compounds, for which hydrochromic and thermochromic materials have extensively been used in recent decades. A wide range of materials, such as organic and inorganic metal complexes, polymer nanoparticles, quantum dots, polymer dots, carbon dots, upconverting nanoparticles, and supramolecular structures, could display all of these phenomena depending on their physical and chemical characteristics. The polymeric anticounterfeiting inks have recently received significant attention because of their high stability for printing on confidential documents. In addition, the printing technologies including hand-writing, stamping, inkjet printing, screen printing, and anticounterfeiting labels are discussed for introduction of the most efficient methods for application of different anticounterfeiting inks. This review would help scientists to design and develop the most applicable encryption, authentication, and anticounterfeiting technologies with high security, fast detection, and potential applications in security marking and information encryption on various substrates.

An innovative prussian blue nanocubes decomposition-assisted signal amplification strategy suitable for competitive lateral flow immunoassay to sensitively detect aflatoxin B1

A prussian blue nanocubes decomposition-assisted signal amplification strategy for competitive lateral flow immunoassay (PBNCD-SALFIA) was innovatively proposed to analyze aflatoxin B₁ (AFB₁) in foodstuffs. The signal was amplified on account of the PBNCs can degrade and fade under alkaline condition, which could weak the color of the test- and control-lines, thereby improving the sensitivity of the sensor. The strategy was technically-simple, just using NaOH as an amplifier can realize signal amplification in the competition assay. Under optimized conditions, the enhanced strip exhibited excellent specificity and sensitivity in AFB₁ monitoring with a detection limit of 23 pg/mL, which was approximately 4- and 8-folds lower than those of PBNCs-LFIA (90 pg/mL) and conventional gold nanoparticles-LFIA (175 pg/mL), respectively. Taking the advantage of the color-fading, this platform revealing the lowest detectable concentration of 0.184, 0.368 and 0.184 μg/kg for AFB₁ in corn, peanut and pumpkin seed within 58 min, separately, showing reliable results.