A portable immune-thermometer assay based on the photothermal effect of graphene oxides for the rapid detection of Salmonella typhimurium

Overview of the Design and Application of Photothermal Immunoassays

Developing powerful immunoassays for sensitive and real-time detection of targets has always been a challenging task. Due to their advantages of direct readout, controllable sensing, and low background interference, photothermal immunoassays have become a type of new technology that can be used for various applications such as disease diagnosis, environmental monitoring, and food safety. By modification with antibodies, photothermal materials can induce temperature changes by converting light energy into heat, thereby reporting specific target recognition events. This article reviews the design and application of photothermal immunoassays based on different photothermal materials, including noble metal nanomaterials, carbon-based nanomaterials, two-dimensional nanomaterials, metal oxide and sulfide nanomaterials, Prussian blue nanoparticles, small organic molecules, polymers, etc. It pays special attention to the role of photothermal materials and the working principle of various immunoassays. Additionally, the challenges and prospects for future development of photothermal immunoassays are briefly discussed.

Synthesis of dual models multivalent activatable aptamers based on HCR and RCA for ultrasensitive detection of Salmonella typhimurium

Aptamers have superior structural properties and have been widely used in bacterial detection methods. However, the problem of low affinity still exists in complex sample detection. In contrast, hybridization chain reaction (HCR)-based model I and rolling circle amplification (RCA)-based model II multivalent activatable aptamers (multi-Apts) can fulfill the need for low-cost, rapid, highly sensitive and high affinity detection of S. typhimurium. In our research, two models of multi-Apts were designed. First, a monovalent activatable aptamer (mono-Apt) was constructed by fluorescence resonance energy transfer (FRET) with an S. typhimurium aptamer and its complementary chain of BHQ1. Next, the DNA scaffold was obtained by HCR and RCA, and the multi-Apts were obtained by self-assembly of the mono-Apt with a DNA scaffold. In model I, when target was presented, the complementary chain BHQ1 was released due to the binding of multi-Apts to the target and was subsequently adsorbed by UIO66. Finally, a FRET-based fluorescence detection signal was obtained. In mode II, the multi-Apts bound to the target, and the complementary chain BHQ1 was released to become the trigger chain for the next round of amplification of HCR with a fluorescence detection signal. HCR and RCA based multi-Apts were able to detect S. typhimurium as low as 2 CFU mL-1 and 1 CFU mL-1 respectively. Multi-Apts amplification strategy provides a new method for early diagnosis of pathogenic microorganisms in foods.

A biphasic accelerated strand exchange amplification strategy for culture-independent and rapid detection of Salmonella enterica in food samples

Salmonella enterica is a common foodborne pathogen that can cause food poisoning in humans. The organism also infects and causes disease in animals. Rapid and sensitive detection of S. enterica is essential to prevent the spread of this pathogen. Traditional technologies for the extraction and detection of this pathogen from complex food matrices are cumbersome and time-consuming. In this study, we introduced a novel strategy of biphasic assay integrated with an accelerated strand exchange amplification (ASEA) method for efficient detection of S. enterica without culture or other extraction procedures. Food samples are rapidly dried, resulting in a physical fluidic network inside the dried food matrix, which allows polymerases and primers to access the target DNA and initiate ASEA. The dried food matrix is defined as the solid phase, while amplification products are enriched in the supernatant (liquid phase) and generate fluorescence signals. The analytical performances demonstrated that this strategy was able to specifically identify S. enterica and did not show any cross-reaction with other common foodborne pathogens. For artificially spiked food samples, the strategy can detect 5.0 × 101 CFU mL-1S. enterica in milk, 1.0 × 102 CFU g-1 in duck, scallop or lettuce, and 1.0 × 103 CFU g-1 in either oyster or cucumber samples without pre-enrichment of the target pathogen. We further validated the strategy using 82 real food samples, and this strategy showed 92% sensitivity. The entire detection process can be finished, sample-to-answer, within 50 min, dramatically decreasing the detection time. Therefore, we believe that the proposed method enables rapid and sensitive detection of S. enterica and holds great promise for the food safety industry.

Application of photothermal effects of nanomaterials in food safety detection

Numerous nanomaterials endowed with outstanding light harvesting and photothermal conversion abilities have been extensively applied in various fields, such as photothermal diagnosis and therapy, trace substance detection, and optical imaging. Although photothermal detection methods have been established utilizing the photothermal effect of nanomaterials in recent years, there is a scarcity of reviews regarding their application in food safety detection. Herein, the recent advancements in the photothermal conversion mechanism, photothermal conversion efficiency calculation, and preparation method of photothermal nanomaterials were reviewed. In particular, the application of photothermal nanomaterials in various food hazard analyses and the newly established photothermal detection methods were comprehensively discussed. Moreover, the development and promising future trends of photothermal nanomaterial-based detection methods were discussed, which provide a reference for researchers to propose more effective, sensitive, and accurate detection methods.

A gas-driven capillary based on the synergy of the catalytic and photothermal effect of PB@Au for Salmonella typhimurium detection

Rapid detection method for Salmonella typhimurium is vital to prevent the spread of food-borne diseases. In this work, a gas-driven capillary detection method was established to achieve sensitive and rapid detection of Salmonella typhimurium using the catalytic and photothermal synergy of Prussian blue-nanogold (PB@Au) nanomaterials. The immuno-PB@Au probe attached to the capillary by specific identification of target bacteria catalyzed the H2O2 under laser irradiation, driving the H2O2 liquid column to move (ΔL) by producing gas, and achieving the quantitative detection of Salmonella typhimurium. After detailed optimization of the critical performance parameters of the gas-driven capillary assay, the limit of detection (LOD) after laser irradiation and being catalyzed by PB@Au was calculated to be 37 CFU mL-1 through the determination of different concentrations of target bacteria. Furthermore, the detection performances of the gas-driven capillary method were evaluated in detail, and the recoveries ranging from 92.9 ± 4.7 % to 107.7 ± 4.1 % were achieved using the spiked actual samples with complex matrices, indicating that the established rapid assay can offer promising strategies for the monitoring and controlling of food-borne pathogenic bacteria.

Investigating enzyme kinetics and fluorescence sensing strategy of CRISPR/Cas12a for foodborne pathogenic bacteria

Foodborne pathogenic bacteria are widespread in various foods, whose cross-contamination and re-contamination are critical influences on food safety. Rapid, accurate, and sensitive detection of foodborne pathogenic bacteria remains a topic of concern. CRISPR/Cas12a can recognize double-stranded DNA directly, showing great potential in nucleic acid detection. However, few studies have investigated the cleavage properties of CRISPR/Cas12a. In this study, the trans-cleavage properties of LbCas12a and AsCas12a were investigated to construct the detection methods for foodborne pathogenic bacteria. The highly sensitive fluorescent strategies for foodborne pathogens were constructed by analyzing the cleavage rates and properties of substrates at different substrate concentrations. Cas12a was activated in the presence of foodborne pathogenic target sequence was present, resulting in the cleavage of a single-stranded reporter ssDNA co-labelled by fluorescein quencher and fluorescein. The sensitivity and specificity of the Cas12a fluorescent strategy was investigated with Salmonella and Staphylococcus aureus as examples. The results showed that AsCas12a was slightly more capable of trans-cleavage than LbCas12a. The detection limits of AsCas12a for Salmonella and Staphylococcus aureus were 24.9 CFU mL-1 and 1.50 CFU mL-1, respectively. In all the seven bacteria, Staphylococcus aureus and Salmonella were accurately discriminated. The study provided a basis for constructing and improving the CRISPR/Cas12a fluorescence strategies. The AsCas12a-based detection strategy is expected to be a promising method for field detection.

Diagnosis of Neglected Tropical Zoonotic Disease, Leptospirosis in a Clinical Sample Using a Photothermal Immunosensor

Photothermal biosensing based on nanomaterials has gained increasing attention because of its universality and simplicity. Diagnostics of neglected tropical diseases (NTDs) in low-resource settings are challenging in terms of speed, accuracy, and cost-effectiveness. By exploiting the photothermal property of carbon nanotubes (CNTs), simple thermometric measurements can be used to generate quantitative biochemical readouts. Herein, a photothermal immunosensor for leptospirosis detection based on a CNT-labeled monoclonal antibody is established through the sensitive monitoring of the target biomarker LipL32 with a simple thermometer. Under optimum conditions, a linear range up to 106 pg/mL with a limit of detection (LOD) of 300 fg/mL was obtained. Overall, the proposed immunoassay exhibited good precision, selectivity, and acceptable stability. Clinical patient sample analysis with the photothermal sensor proved the differential diagnosis of leptospirosis along with other febrile illnesses. On the other hand, we have also characterized the photothermal sensor platform with surface morphological and spectral techniques to confirm the robust and successful fabrication of the immunosensor. The fabricated photothermal sensor could be used as a potential diagnostic tool for the early detection of NTDs in patients from resource-limited settings, as it does not require sample pretreatment, sophisticated equipment, or skilled labor. Moreover, the developed photothermal assay follows ASSURED criteria, very crucial for diagnosis in resource-limited settings.

Rapid detection of Salmonella in food matrices by photonic PCR based on the photothermal effect of Fe3O4

Salmonella causes most deaths from diarrheal disease worldwide. Therefore, Salmonella must be accurately and quickly detected, even in complex food matrices, which is difficult to achieve using conventional culture methods. Here we propose a novel photonic polymerase chain reaction (PCR) method based on ferroferric oxide (Fe3O4) for the detection of Salmonella typhimurium in complex samples. Owing to the great photothermal conversion performance of Fe3O4, rapid thermal cycling could be accomplished. Our optimized photonic PCR system specifically detected Salmonella typhimurium in complex food matrices within 50 min. Quantitative data showed a limit of detection up to 102 CFU/mL in food samples. This method is suitable for the detection of all pathogenic microorganisms and is universal.

A pressure-colorimetric multimode system with photothermal activated multiple rolling signal amplification for ovarian cancer biomarker detection

Utilizing the photothermal effect to activate enzyme activity, realize signal conversion and amplification show promising prospects in biosensing. Herein, a pressure-colorimetric multi-mode bio-sensor was proposed through the multiple rolling signal amplification strategy of photothermal control. Under NIR light radiation, the Nb₂C MXene labeled photothermal probe caused notable temperature elevation on a multi-functional signal conversion paper (MSCP), leading to decomposition of thermal responsive element and in-situ formation of Nb₂C MXene/Ag-S_x hybrid. The generation of Nb₂C MXene/Ag-S_x hybrid accompanied with valid color change from pale yellow to dark brown on MSCP. Moreover, the Ag-S_x as a signal amplification element enhanced the NIR light absorption to further improve the photothermal effect of Nb₂C MXene/Ag-S_x thereby induce cyclic in situ production of Nb₂C MXene/Ag-S_x hybrid with rolling enhanced photothermal effect. Subsequently, the continuously enhanced photothermal effect rolling activated catalase-like activity of Nb₂C MXene/Ag-S_x, which accelerated the decomposition of H₂O₂ and promoted the pressure elevation. Therefore, the rolling-enhanced photothermal effect and rolling activated catalase-like activity of Nb₂C MXene/Ag-S_x considerately amplified the pressure and color change. Making full use of multi-signal readout conversion and rolling signal amplification, accurate results can be obtained in a short time, whether in the laboratory or in the patient's homes.

DNA-Driven Photothermal Amplification Transducer for Highly Sensitive Visual Determination of Extracellular Vesicles

Extracellular vesicles (EVs) are crucial focus of current biomedical research and future medical diagnosis. However, the requirement for specialized sophisticated instruments for quantitative readouts has limited the sensitive measurement of EVs to specialized laboratory settings, which in turn has limited bench-to-bedside translation of EV-based liquid biopsies. In this work, a straightforward temperature-output platform based on a DNA-driven photothermal amplification transducer was developed for the highly sensitive visual detection of EVs using a simple household thermometer. The EVs were specifically recognized by the antibody-aptamer sandwich immune-configuration that was constructed on portable microplates. Via a one-pot reaction, cutting-mediated exponential rolling circle amplification was initiated in situ on the EV surface, generating substantial G-quadruplex-DNA-hemin conjugates. Significant amplification in temperature was achieved from the effective photothermal conversion and regulation guided by the G-quadruplex-DNA-hemin conjugates in the 3,3',5,5'-tetramethylbenzidine-H2O2 system. Through obvious temperature outputs, the DNA-driven photothermal transducer enabled highly sensitive EV detection at close to the single-particle level and supported the highly specific identification of tumor-derived EVs directly in serum samples, without the requirement of any sophisticated instrument or labeling process. Benefiting from highly sensitive visual quantification, an easy-to-use readout, and portable detection, this photothermometric strategy is expected to be deliverable across professional on-site screening to home self-testing as EV-based liquid biopsies.

Isolation of Bacteria Aptamers with Non-SELEX for the Development of a Highly Sensitive Colorimetric Assay Based on Dual Signal Amplification

In this work, an aptamer against Escherichia coli is isolated via non-SELEX, which executes efficient selection by employing repetitive cycles of centrifugation-based partitioning, and the binding site of the aptamer on E. coli cell surfaces is inferred to be a membrane protein. Moreover, truncated sequence 2-17-2 with a higher affinity (K_d = 101.76 nM) is employed for highly sensitive colorimetric detection of bacteria based on the dual signal amplification strategy. When targets exist, the release of DNA 1 from the polymer activates a hybridization chain reaction (HCR) between DNA 1 and DNA 2, thereby inducing the aggregation of probe 1. Subsequently, DNA 3 dissociated from probe 1 as a linker DNA further assembles probe 2/3. In this system, two types of DNA@gold nanoparticles (AuNPs) coexist and successively aggregate AuNPs based on divergent triggering mechanisms. Under optimal conditions, the dual signal amplification strategy presents excellent sensitivity (10 CFU mL^-1) and specificity, as well as the realization of real sample analysis.

Microfluidic platforms integrated with nano-sensors for point-of-care bioanalysis

Microfluidic technology provides a portable, cost-effective, and versatile tool for point-of-care (POC) bioanalysis because of its associated advantages such as fast analysis, low volumes of reagent consumption, and high portability. Along with microfluidics, the application of nanomaterials in biosensing has attracted lots of attention due to their unique physical and chemical properties for enhanced signal modulation such as signal amplification and signal transduction for POC bioanalysis. Hence, an enormous number of microfluidic devices integrated with nano-sensors have been developed for POC bioanalysis targeting low-resource settings. Herein, we review recent advances in POC bioanalysis on nano-sensor-based microfluidic platforms. We first briefly summarized the different types of cost-effective microfluidic platforms, followed by a concise introduction to nanomaterial-based biosensors. Then, we highlighted the application of microfluidic platforms integrated with nano-sensors for POC bioanalysis. Finally, we discussed the current limitations and perspective trends of the nano-sensor-based microfluidic platforms for POC bioanalysis.

Aptamer-AuNP-conjugated carboxymethyl chitosan-functionalized graphene oxide for colorimetric identification of Salmonella typhimurium

A novel aptamer-AuNP-conjugated carboxymethyl chitosan-functionalized graphene oxide (CMC/GO@Apt-Au NP) probe was for the first time developed for the determination of Salmonella typhimurium (S. typhimurium). Owing to the conformational change of the aptamers in the presence of S. typhimurium, the Au NPs, which were pre-adsorbed on the aptamers through van der Waals forces, were released into the solution phase and induced the color change of the solution. As a result, S. typhimurium ranging from 10² to 10⁷ CFU/mL was successfully identified using the designed assay with a limit of detection (LOD) of 10 CFU/mL. This low detection level allowed the sensitive recognition of S. typhimurium in milk samples within 40 min without sample pretreatment, a conclusion that agreed well with the traditional plate counting method. The developed method not only provides a rapid way for the determination of S. typhimurium with simplicity and sensitivity but also shows potential universality in the quantification of other pathogenic microorganisms.

Salmonella typhimurium strip based on the photothermal effect and catalytic color overlap of PB@Au nanocomposite

This work reports a sensitive and accurate multimode detection method to detect Salmonella typhimurium using inherent color, photothermal and catalytic properties of Prussian blue@gold nanoparticles (PB@Au). The inherent color of PB@Au can realize direct visual detection while the temperature increase (ΔT) of it can realize sensitive and quantitative photothermal detection. Moreover, catalytic coloration detection is applied to further amplify detection signal. The risk limit, prevention and control of Salmonella typhimurium can be more intuitively displayed through catalytic color overlap degree between PB@Au and catalytic product. The sensitivity of method is improved through photothermal and catalytic coloration detection (10¹ CFU·mL^-1) compared with direct visual detection (10² CFU·mL^-1). The multimode detection improves the accuracy of method, and exhibits good repeatability, acceptable selectivity and stability. This method is also successfully applied in real samples, displaying its good practical applicability.

Diagnosis of disease relevant nucleic acid biomarkers with off-the-shelf devices

Changes in the level of nucleic acids in blood may be correlated with some clinical disorders like cancer, stroke, trauma and autoimmune diseases, and thus, nucleic acids can serve as potential biomarkers for pathological processes. The requirement of technical equipment and operator expertise in effective information readout of modern molecular diagnostic technologies significantly restricted application outside clinical laboratories. The ability to detect nucleic acid biomarkers with off-the-shelf devices, which have the advantages of portability, simplicity, low cost and short response time, is critical to provide a prompt clinical result in circumstances where the laboratory instruments are not available. This review throws light on the current strategies and challenges for nucleic acid diagnosis with commercial portable devices, indicating the future prospect of portable diagnostic devices and making a great difference in improving the healthcare and disease surveillance in resource-limited areas.

Two-stage nicking enzyme signal amplification (NESA)-based biosensing platform for the ultrasensitive electrochemical detection of pathogenic bacteria

The sensitive and selective detection of pathogenic bacteria represents an essential approach in food safety analysis and clinical diagnostics. We report the development of a simple, rapid, and low-cost electrochemical biosensing strategy for the detection of pathogenic bacteria with ultrasensitivity and high specificity. The biosensor relies on the target and aptamer binding-triggered two-stage nicking enzyme signal amplification (NESA) and three-way junction probe-mediated electrochemical signal transduction. In the presence of the target S. typhimurium, the specific binding of S. typhimurium and aptamer results in the release of a primer, which hybridizes with HAP1 and initiates an extension reaction with the aid of polymerase and dNTPs. A specific recognition site for Nt.BsmaI is generated in the DNA duplex; thus, the produced DNA is nicked and the secondary primer is released (named recycle I). Subsequently, the reaction solution supplemented with a helper DNA is dropped on the electrode surface, and a three-way junction probe containing a specific recognition site for Nt.BsmaI is thus formed. The MB-labeled probe is nicked with the help of Nt.BsmaI and the dissociated primer-helper DNA duplex combines with another HAP2 (named recycle II). Thus, a remarkably decreased electrochemical signal is generated because the electroactive MB is far away from the electrode surface. As far as we know, this work is the first time that NESA and three-way junction probe-mediated electrochemical signal transduction has been used for pathogenic bacteria detection. Under optimal conditions, the results reveal that the calibration plot obtained for S. typhimurium is approximately linear from 9.6 to 9.6 × 10⁵ cfu mL^-1 with the limit of detection of 8 cfu mL^-1. Additionally, the proposed strategy has been successfully applied to the quantitative assay of S. typhimurium in the real samples. Therefore, the NESA-based biosensing strategy might create a useful and practical platform for pathogenic bacteria identification, and the related food safety analysis and clinical diagnosis.

AuAg nanocages/graphdiyne for rapid elimination and detection of trace pathogenic bacteria

We prepared a biocompatible AuAg nanocages/graphdiyne @ polyethylene glycol (AuAg/GDY@PEG) composite. The combination of AuAg and GDY to obtain a synergistically enhanced photothermal effect, and the antibacterial effect of GDY and AuAg are used in combined anti-infective therapy. The in vitro antibacterial activity of AuAg/GDY@PEG was investigated, showing an impressive broad-spectrum antibacterial activity with the killing rate > 99.999%. Based on the photothermal conversion ability of AuAg/GDY@PEG, a simple photothermal immunoassay for pathogenic bacteria was successfully established. Sandwich immune response was performed on a microporous plate, the microplate containing the antibody binds specifically to the bacterium being tested, which then binds to the material with the antibody on its surface, and the signal was a change in temperature under 808 nm near-infrared light. The limit of detection (LOD) for S. typhimurium detection is 10³ CFU mL^-1, with a range of 10³-10⁷ CFU mL^-1. This method is accurate, rapid and low-cost, which can be used for on-site detection of pathogenic bacteria in food.

A polypyrrole-mediated photothermal biosensor with a temperature and pressure dual readout for the detection of protein biomarkers

A novel photothermal biosensor with a temperature and pressure dual readout was developed for CRP detection. The in situ synthesized polypyrrole exhibits photothermal effect under NIR light to increase temperature and pressure for portable readout.

New analytical methods using carbon-based nanomaterials for detection of Salmonella species as a major food poisoning organism in water and soil resources

Salmonella is one of the most prevalent causing agents of food- and water-borne illnesses, posing an ongoing public health threat. These food-poisoning bacteria contaminate the resources at different stages such as production, aggregation, processing, distribution, as well as marketing. According to the high incidence of salmonellosis, effective strategies for early-stage detection are required at the highest priority. Since traditional culture-dependent methods and polymerase chain reaction are labor-intensive and time-taking, identification of early and accurate detection of Salmonella in food and water samples can prevent significant health economic burden and lessen the costs. The immense potentiality of biosensors in diagnosis, such as simplicity in operation, the ability of multiplex analysis, high sensitivity, and specificity, have driven research in the evolution of nanotechnology, innovating newer biosensors. Carbon nanomaterials enhance the detection sensitivity of biosensors while obtaining low levels of detection limits due to their possibility to immobilize huge amounts of bioreceptor units at insignificant volume. Moreover, conjugation and functionalization of carbon nanomaterials with metallic nanoparticles or organic molecules enables surface functional groups. According to these remarkable properties, carbon nanomaterials are widely exploited in the development of novel biosensors. To be specific, carbon nanomaterials such as carbon nanotubes, graphene and fullerenes function as transducers in the analyte recognition process or surface immobilizers for biomolecules. Herein the potential application of carbon nanomaterials in the development of novel Salmonella biosensors platforms is reviewed comprehensively. In addition, the current problems and critical analyses of the future perspectives of Salmonella biosensors are discussed.

Trigging Isothermal Circular Amplification-Based Tuning of Rigorous Fluorescence Quenching into Complete Restoration on a Multivalent Aptamer Probe Enables Ultrasensitive Detection of Salmonella

Detection of pathogenic bacteria is of vital significance for combating and preventing infectious diseases. In this work, we developed a multivalent aptamer probe (Multi-VAP)-based trigging isothermal circular amplification (TICA) for rapidly and ultrasensitively detecting Salmonella. In this sensing system, the fluorescence of Multi-VAP was strongly quenched via the dual effect of FRET. Introduction of Salmonella to the system forced the configuration change of Multi-VAP, leading to the occurrence of a TICA responsible for tuning all of the fluorescence-quenched Multi-VAP into a complete restoration state. This prominent feature allows the reasonable combination of a strong background restraint and great target signal amplification into one sensing system, which in turn benefits the improvement of the signal-to-noise ratio to ensure that the system has an ultrahigh sensitivity. Combined with the employment of an aptamer to ensure that it has excellent specificity, the Salmonella can be quantitatively and qualitatively analyzed even from human serum. The total processing merely requires sample addition and incubation. The turnaround time of the complete analysis from "sample-to-result" was within 30 min. With the method to decrease the time to detect and simplify the process to operate, the assay was successfully used as a sensing platform for specific detection of as few as 9 CFU/mL Salmonella.

Integration and Quantitative Visualization of 3,3',5,5'-Tetramethylbenzidine-Probed Enzyme-Linked Immunosorbent Assay-like Signals in a Photothermal Bar-Chart Microfluidic Chip for Multiplexed Immunosensing

The photothermal effect shows significant promise for various biomedical applications but is rarely exploited for microfluidic lab-on-a-chip bioassays. Herein, a photothermal bar-chart microfluidic immunosensing chip, with the integration of the conventional 3,3',5,5'-tetramethylbenzidine (TMB)-probed enzyme-linked immunosorbent assay (ELISA)-like system, was developed based on exploiting the photothermal pumping technique for visual bar-chart microfluidic immunosensing. Both the sandwich ELISA-like system and the photothermal pumping protocol were integrated into a single photothermal bar-chart chip. On-chip immunocaptured iron oxide nanoparticles catalyzed the oxidation of the chromogenic substrate, TMB, to produce a sensitive photothermal and chromogenic dual-functional probe, oxidized TMB. As the result of heat generation and the subsequent production of elevating vapor pressure in the sealed microfluidic environment, the on-chip near-infrared laser-driven photothermal effect of the probe served as a dose-dependent pumping force to drive the multiplexed quantitative display of the immunosensing signals as visual dye bar charts. Prostate-specific antigen as a model analyte was tested at a limit of detection of 1.9 ng·mL^-1, lower than the clinical diagnostic threshold of prostate cancer. This work presents a new perspective for microfluidic integration and multiplexed quantitative bar-chart visualization of the conventional TMB-probed ELISA signals possibly by means of an affordable handheld laser pointer in a lab-on-a-chip format.

Overview of Rapid Detection Methods for Salmonella in Foods: Progress and Challenges

Salmonella contamination in food production and processing is a serious threat to consumer health. More and more rapid detection methods have been proposed to compensate for the inefficiency of traditional bacterial cultures to suppress the high prevalence of Salmonella more efficiently. The contamination of Salmonella in foods can be identified by recognition elements and screened using rapid detection methods with different measurable signals (optical, electrical, etc.). Therefore, the different signal transduction mechanisms and Salmonella recognition elements are the key of the sensitivity, accuracy and specificity for the rapid detection methods. In this review, the bioreceptors for Salmonella were firstly summarized and described, then the current promising Salmonella rapid detection methods in foodstuffs with different signal transduction were objectively summarized and evaluated. Moreover, the challenges faced by these methods in practical monitoring and the development prospect were also emphasized to shed light on a new perspective for the Salmonella rapid detection methods applications.

A sensitive photothermometric biosensor based on redox reaction-controlled nanoprobe conversion from Prussian blue to Prussian white

As a new low-cost photothermal nanoprobe, Prussian blue nanoparticles (PB NPs) have been demonstrated to have more potential in photothermometric-based point-of-care testing (POCT) application. However, most of the existing PB NP-based photothermometric sensors were constructed mainly relying on in situ generation of PB NPs or their combination with antigens and antibodies, therefore usually suffering from the inherent defects like complicated preparation and cumbersome surface process as well as high-cost modification. To break this limitation of PB NP-based photothermometric POCT, we proposed an ingenious redox reaction-controlled nanoprobe conversion strategy and successfully applied to photothermometric detection of ascorbate oxidase (AAO). In this design, the heat of PB NP photothermal system under 808-nm laser irradiation dramatically decreased with the addition of AA, due to a unique AA-induced Prussian blue to Prussian white (PB-to-PW) conversion. Upon AAO addition, the heat of reaction system increased because of the enzymatic catalytic reaction between AAO and AA, which led to a significant reduction of AA and resultantly inhibited PB-to-PW conversion. Such target-mediated nanoprobe conversion resulted in an obvious temperature change that could be easily detected by a common thermometer and exhibited good linear ranges from 0.25 to 14 mU/mL with a detection limit as low as 0.21 mU/mL for POCT analysis of AAO. This facile, convenient, and portable photothermometric sensing platform provides an innovative route for the design of PB NP nanoprobe-based photothermometric detection methods. A sensitive photothermometric AAO sensor based on a redox reaction-controlled nanoprobe conversion strategy from Prussian blue to Prussian white.

Surface-enhanced Raman spectroscopy for comparison of serum samples of typhoid and tuberculosis patients of different stages

BACKGROUND

Surface-enhanced Raman spectroscopy (SERS) is a reliable tool for the identification and differentiation of two different human pathological conditions sharing the same symptomology, typhoid and tuberculosis (TB).

OBJECTIVES

To explore the potential of surface-enhanced Raman spectroscopy for differentiation of two different diseases showing the same symptoms and analysis by principal component analysis (PCA) and partial least square discriminate analysis (PLS-DA).

METHODS

Serum samples of clinically diagnosed typhoid and tuberculosis infected individuals were analyzed and differentiated by SERS using silver nanoparticles (Ag NPs) as a SERS substrate. For this purpose, the collected serum samples were analyzed under the SERS instrument and unique SERS spectra of typhoid and tuberculosis were compared showing notable spectral differences in protein, lipid and carbohydrates features. Different stages of the diseased class of typhoid (Early acute and late acute stage) and tuberculosis (Pulmonary and extra-pulmonary stage) were compared with each other and with healthy human serum samples, which were significantly separated. Moreover, SERS data was analyzed using multivariate data analysis techniques including principal component analysis (PCA) and partial least square discriminate analysis (PLS-DA) and differences were so prominent to observe.

RESULTS

SERS Spectral data of typhoid and tuberculosis showed clear differences and were significantly separated using PCA. SERS spectral data of both stages of typhoid and tuberculosis were separated according to 1st principle component. Moreover, by analyzing data using partial least square discriminate analysis, differentiation of two disease classes were considered more valid with a 100% value of sensitivity, specificity and accuracy.

CONCLUSION

SERS can be employed for identification and comparison of two different human pathological conditions sharing same symptomology.

Detector-Free Photothermal Bar-Chart Microfluidic Chips (PT-Chips) for Visual Quantitative Detection of Biomarkers

The volumetric bar-chart microfluidic chips (V-Chips) driven by chemical reaction-generated gas provide a promising platform for point-of-care (POC) visual biomarker quantitation. However, multiple limitations are encountered in conventional V-Chips, such as costly and complex chip fabrication, complicated chip assembly, and imprecise controllability of gas production. Herein, we introduced nanomaterial-mediated photothermal effects to V-Chips, and for the first time developed a new type of V-Chip, photothermal bar-chart microfluidic chip (PT-Chip), for visual quantitative detection of biochemicals without any bulky and costly analytical instruments. Immunosensing signals were converted to visual readout signals via photothermal effects, the on-chip bar-chart movements, enabling quantitative biomarker detection on a low-cost polymer hybrid PT-Chip with on-chip scale rulers. Four different human serum samples containing a prostate-specific antigen (PSA) as a model analyte were detected simultaneously using the PT-Chip, with a limit of detection of 2.1 ng/mL, meeting clinical diagnostic requirements. Although no conventional signal detectors were used, it achieved comparable detection sensitivity to absorbance measurements with a microplate reader. The PT-Chip was further validated by testing human whole blood without the color interference problem, demonstrating the good analytical performance of our method even in complex matrices and thus the potential to fill the gap in current clinical diagnostics that is incapable of testing whole blood. This new PT-Chip driven by nanomaterial-mediated photothermal effects opens a new horizon of microfluidic platforms for instrument-free diagnostics at the point-of-care.

Biosensors for rapid detection of Salmonella in food: A review

Salmonella is one of the main causes of foodborne infectious diseases, posing a serious threat to public health. It can enter the food supply chain at various stages of production, processing, distribution, and marketing. High prevalence of Salmonella necessitates efficient and effective approaches for its identification, detection, and monitoring at an early stage. Because conventional methods based on plate counting and real-time polymerase chain reaction are time-consuming and laborious, novel rapid detection methods are urgently needed for in-field and on-line applications. Biosensors provide many advantages over conventional laboratory assays in terms of sensitivity, specificity, and accuracy, and show superiority in rapid response and potential portability. They are now recognized as promising alternative tools and one of the most on-site applicable and end user-accessible methods for rapid detection. In recent years, we have witnessed a flourishing of studies in the development of robust and elaborate biosensors for detection of Salmonella in food. This review aims to provide a comprehensive overview on Salmonella biosensors by highlighting different signal-transducing mechanisms (optical, electrochemical, piezoelectric, etc.) and critically analyzing its recent trends, particularly in combination with nanomaterials, microfluidics, portable instruments, and smartphones. Furthermore, current challenges are emphasized and future perspectives are discussed.

Multifunctional nanoplatform for dual-mode sensitive detection of pathogenic bacteria and the real-time bacteria inactivation

Bacterial infection is a growing public health concern and causes a huge medical and financial burden. It is of significance to efficiently construct multifunctional platforms for bacterial point-of-care testing (POCT) and elimination. Herein, near-infrared (NIR) light-responded vancomycin-doped prussian blue nanoparticles (PB-VANNPs) with high efficient photothermal conversion was synthesized for binding, dual-mode portable detection, and elimination of bacteria. The PB-VANNPs can bind to the surface of Gram-positive bacteria such as Staphylococcus aureus (S. aureus), forming complex of PB-VANNPs/S. aureus. After being centrifugated, the suspension solution of PB-VANNPs can stimulate perfluorohexane (PFH) to rapidly release oxygen (O2) under NIR irradiation. Thus, the bacteria can be sensitively detected with portable pressure meter as signal reader, reporting a limit of detection (LOD) of 1.0 CFU mL-1. On the other side, the sediment of PB-VANNPs/S. aureus can be detected via thermal camera, reporting a LOD of 1.0 CFU mL-1. Interestingly, the bacteria can be effectively inactivated with the local temperature elevation during temperature-based detection. The antibacterial efficiency reaches as high as 99.8%. The developed multifunctional nanoplatform not only provides a straightforward "mix-then-test" way for portable detection of bacteria with high sensitivity, also realizes high efficiency elimination of bacteria simultaneously. The developed strategy was further applied for promoting wound healing of bacteria-infected mice.

Salmonella typhimurium detector based on the intrinsic peroxidase-like activity and photothermal effect of MoS2

A multimode dot-filtration immunoassay (MDFIA) was established for rapid and accurate detection of the target (Salmonella typhimurium), which was based on the intrinsic color, peroxidase-like activity and photothermal effect of molybdenum disulfide (MoS₂). Obviously, multimode detection can improve detection accuracy compared to the direct visual detection in test strips. A thermal imaging camera was used as detector to record the temperature change (ΔT) of MoS₂ and establish the standard curve of ΔT and the concentration of Salmonella typhimurium to realize quantitative determination. The main parameters that affect the analytical performance of MDFIA were optimized. Under the optimal experimental conditions, the limit of detection (LOD) of photothermal detection reached 10² CFU mL^-1 and was one order of magnitude lower than the limit of direct visual detection and catalytic color development detection (10³ CFU mL^-1). The accuracy and analytical sensitivity were enhanced by intrinsic peroxidase-like activity and the huge photothermal effect of MoS₂. Moreover, this method exhibited high selectivity, good repeatability, and acceptable stability and the entire process was simple to be accomplished in 30 min, which generally meets the need of rapid detection. The successful implementation in real samples with the recovery being between 99.5 and 119.2% showed that it could be used as a promising quality control strategy for detection of other foodborne pathogens. The peroxidase-like activity and excellent photothermal effect of MoS₂ was used to develop a multimode dot-filtration immunoassay for rapid detection of Salmonella typhimurium.

Low-Cost Quantitative Photothermal Genetic Detection of Pathogens on a Paper Hybrid Device Using a Thermometer

Tuberculosis (TB), one of the deadliest infectious diseases, is caused by Mycobacterium tuberculosis (MTB) and remains a public health problem nowadays. Conventional MTB DNA detection methods require sophisticated infrastructure and well-trained personnel, which leads to increasing complexity and high cost for diagnostics and limits their wide accessibility in low-resource settings. To address these issues, we have developed a low-cost photothermal biosensing method for the quantitative genetic detection of pathogens such as MTB DNA on a paper hybrid device using a thermometer. First, DNA capture probes were simply immobilized on paper through a one-step surface modification process. After DNA sandwich hybridization, oligonucleotide-functionalized gold nanoparticles (AuNPs) were introduced on paper and then catalyzed the oxidation reaction of 3,3',5,5'-tetramethylbenzidine (TMB). The produced oxidized TMB, acting as a strong photothermal agent, was used for the photothermal biosensing of MTB DNA under 808 nm laser irradiation. Under optimal conditions, the on-chip quantitative detection of the target DNA was readily achieved using an inexpensive thermometer as a signal recorder. This method does not require any expensive analytical instrumentation but can achieve higher sensitivity and there are no color interference issues, compared to conventional colorimetric methods. The method was further validated by detecting genomic DNA with high specificity. To the best of our knowledge, this is the first photothermal biosensing strategy for quantitative nucleic acid analysis on microfluidics using a thermometer, which brings fresh inspirations on the development of simple, low-cost, and miniaturized photothermal diagnostic platforms for quantitative detection of a variety of diseases at the point of care.

Multiplexed tri-mode visual outputs of immunoassay signals on a clip-magazine-assembled photothermal biosensing disk

The photothermal biosensing principle is of increasing interest for point-of-care detection, but has rarely been applied in portable analytical devices in a lab-on-a-chip format. Herein, a photothermally responsive poly (methyl methacrylate) (PMMA)/paper hybrid disk (PT-Disk) was developed as a novel photothermal immunoassay device with the integration of a clip-magazine-assembled photothermal biosensing strategy. The PT-Disk consisted of a dissociative thermoresponsive hydrogel-loaded clip unit where the sandwich-type immunoreaction with an iron oxide-to-Prussian blue nanoparticle (PB NP) conversion took place and a magazine bearer for the rotational clip assembly and visual signal outputs. Upon laser irradiation of the clip-magazine-assembled PT-Disk, on-chip photothermal effect of PB NPs triggered both dose-dependent temperature elevation and the subsequent release of dye solutions from the central clip unit to surrounding magazine-bearing paper channels as the result of phase transition of the hydrogels, realizing multiplexed thermal image- and distance-based visual quantitative signal outputs in combination with the preliminary colorimetric readout on the PT-Disk. Using the multiplexed tri-mode signal outputs, the PT-Disk can quantify prostate specific antigen with limits of detection of 1.4-2.8 ng mL^-1. This is the first attempt to apply the photothermal biosensing principle in portable PMMA/paper-based analytical devices, which offers not only versatile on-chip visual quantitative signal outputs, but also the implementation of the photothermal biosensing principle in a lab-on-a-chip format.

A sensitive biosensor for determination of pathogenic bacteria using aldehyde dehydrogenase signaling system

Aldehyde dehydrogenase (ALDH) was first developed as an enzymatic signaling system of a biosensor for sensitive point-of-care detection of pathogenic bacteria. ALDH and specific aptamers to Salmonella typhimurium (S. typhimurium), as organic components, were embedded in organic-inorganic nanocomposites as a biosensor signal label, integrating the functions of signal amplification and target recognition. The biosensing mechanism is based on the fact that ALDH can catalyze rapid oxidation of acetaldehyde into acetic acid, resulting in pH change with portable pH meter readout. The altered pH exhibited a linear relationship with the logarithm of S. typhimurium from 10² to 10⁸ CFU/mL and detection limit of 46 CFU/mL. Thus, the proposed biosensor has potential application in the diagnosis of pathogenic bacteria.

From hazard analysis to risk control using rapid methods in microbiology: A practical approach for the food industry

The prevention of foodborne diseases is one of the main objectives of health authorities. To this effect, analytical techniques to detect and/or quantify the microbiological contamination of foods prior to their release onto the market are required. Management and control of foodborne pathogens have generally been based on conventional detection methodologies, which are not only time-consuming and labor-intensive but also involve high consumable materials costs. However, this management perspective has changed over time given that the food industry requires efficient analytical methods that obtain rapid results. This review covers the historical context of traditional methods and their passage in time through to the latest developments in rapid methods and their implementation in the food sector. Improvements and limitations in the detection of the most relevant pathogens are discussed from a perspective applicable to the current situation in the food industry. Considering efforts that are being done and recent developments, rapid and accurate methods already used in the food industry will be also affordable and portable and offer connectivity in near future, which improves decision-making and safety throughout the food chain.