Application Of Visual Sensors Based On Noble Metal Nanomaterials

Visualization detection methods are used for qualitative or quantitative detection of the target by comparing the optical intensity or color change of the reaction solution with the naked-eye.Visualization detection method has obvious advantages of simple operation,results visualization and no need of any complicated equipment.With the rapid development of nanotechnology,noble metal nanomaterials have been widely explored for the construction of visualization sensors due to their excellent optical properties,high catalytic activity,and easy for surface modification,which greatly improving the stability and sensitivity of sensors.Based on the unique physical and chemical properties of noble metal nanomaterials,this paper develops simple,portable,sensitive visual sensors for detection of the target in many fields,including food safety,disease diagnosis,and environmental monitoring.1)Immunoassays provide ideal systems for implementing a wide range of rapid,simple,and sensitive biochemical detections.Herein,a signal-off tuned signal-on(SF-T-SN)colorimetric immunoassay has been developed for amantadine(AMD)detection.In this strategy,the conventional competitive signal-off immunoassay acts as the foundation to cause different immune recognition events in the absence and presence of AMD.The introduction of activity-metalmodulated peroxidase-mimicking nanozyme then intriguingly converts the conventional signal-off model into a more satisfying signal-on reporting behavior and overcomes the disadvantages of using natural enzymes.Importantly,due to the switching of the reporting mechanism and the high catalytic activity of nanozyme,this“off-to-on”pathway makes the nanozyme-based colorimetric response proportional to exposed AMD concentration,enabling the circumvention of the inherent drawbacks of limited signaling ability and sensitivity of conventional signal-off immunoassay-based small molecules detection.The concentration of the AMD could be thus sensitively known by naked eyes or the measurement of the optical signal.Based on this protocol,the sensitivity(0.195 ng/mL for naked eyes and 0.134 ng/mL for optical detection)of the current one is not only outperformed than reported methods,but it is also improved for at least 60 times and 2 times by naked eyes observation and UV–vis measurements,respectively,when compared with conventional signal-off immunoassay.The AMD-spiked chicken samples can also be detected with acceptable recoveries and relative standard derivations(RSD),indicating the proposed method is applicable for future biochemical analysis and food safety monitoring.2)Terminal deoxynucleotidyl transferase(TdT)is a common biomarker of some diseases,such as leukemia.Therefore,the detection of TdT activity is crucial for the early-stage clinical diagnosis and drug screening.In this paper,we developed a simple,rapid,label-free,and sensitivity multicolor sensor for visual detection of TdT activity based on the etching of Au nanorods(Au NRs)by the oxidation state 3,3',5,5'–tetramethylbenzidine sulfate(TMB²⁺).The extended product of TdT amplification formed a peroxidase-mimic G-quadruplex/Hemin DNAzyme with the help of Hemin and K⁺,subsequently catalyzing the oxidation of TMB by H₂O₂ to produce the etchant TMB²⁺.The Au NRs were etched gradually with the increasing concentration of TdT,resulting in the distinct blue shift of longitudinal LSPR peaks of Au NRs and accompanying a series of color changes.Under the optimal conditions,the visualization sensor can be used for the quantitative detection of TdT activity with a dynamic range of 0.25–12 U,and the limit of detection for TdT activity is 1.0 U observed by the naked-eye and 0.21 U as measured by UV–vis measurement.Compared with traditional colorimetric methods for detecting TdT activity,the most attractive advantage of this sensor is a one-to-one correspondence between the multiple colors display and the concentration of TdT.Meanwhile,the proposed sensor owned high sensitivity and good selectivity,which can easily readout the results without the complicated equipment and suitable for onsite detection in resource-limited areas.3)Aflatoxin B₁(AFB₁),a group of the high toxicity and carcinogenicity food contaminants,are widely existed in agricultural products,food,and edible oils.In this paper,we proposed a high-sensitivity and high-resolution magnetic bead multicolor colorimetric immunoassay(MBMCIA)for detection of ultra-low concentration AFB₁ based silver/gold shell/core nanorods.The method was combined magnetic enzyme-linked immunosorbent assay(MagLISA)with mechanism for the silver shells coated gold nanorods.To design this immunoassay system,magnetic beads(MBs)firstly replaced microplates as immobile phase and prepared the MBs-AFB₁-BSA conjugate by the EDC/NHS reaction;then alkaline phosphatase(ALP),acted as“bridge”between MagLISA system and the chromogenic reaction,can catalyze hydrolyze the sodium ascorbyl phosphate(AA-P)substrate to produce ascorbic acid(AA);finally,the produced AA reduced silver ions and consequent deposition of metallic silver on the gold nanorods,resulting in the changes of longitudinal LSPR peak and solution color.Under the optimal conditions,the color of the solution varied vividly from light reddish-brown to fuchsia,purple,blue,green,and light green with the increase of AFB₁ concentration from 0.01 to 1.00 ng/mL.The limit of detection(LOD)was as low as 5.7 pg/mL,and semi-quantitative detection of lower concentration targets with the naked eye.The recoveries(99.1%–104.3%)and RSD(1.51%–7.05%)indicate that the proposed MBMCIA is suitable and reliable for real samples.The MBMCIA possessing the advances of separation,enrichment,anti-interference,and visual readout may open up a new avenue for onsite visual food safety inspection or environmental monitoring.