Fullerene carbon dot catalytic amplification-aptamer assay platform for ultratrace As+3 utilizing SERS/RRS/Abs trifunctional Au nanoprobes

Fluorogenic target competitors for developing label-free and sensitive folding-unswitching aptamer sensors

BACKGROUND

Aptamers have aroused tremendous applications in sensors, drug deliveries, diagnosis, and therapies. In particular, target-induced global structure switching of aptamers has been widely used to develop selective sensors. However, fluorophore and/or quencher modification, sequence elongation, and nano-interface adsorption are required to design such global structure-switching aptamer sensors (SSAS) in order to signal target binding events. Accordingly, these requirements make SSAS at a high cost and expense of sensors' sensitivity. In this aspect, efforts should be made to overcome these drawbacks of SSAS.

RESULTS

Herein, we tried to develop label-free folding-unswitching aptamer sensors (FUAS) by searching fluorogenic target competitors. Using adenine nucleoside/nucleotide as the proof-of-concept model targets, we screened out berberine (BER) from natural isoquinoline alkaloids (having rings comparable to targets) as the best fluorogenic target competitor. Binding of BER at the conserved nucleotides of intact aptamer foldings turned on this fluorogenic target competitor' fluorescence. Targets then competed with this fluorogenic target competitor over the same conserved nucleotides to cause its release in favor of a resultant fluorescence change. We found that the developed FUAS are much more sensitive than the previously reported SSAS. The FUAS were successfully applied to assays of ATP and adenosine deaminase in serums, and to screening of the adenosine deaminase's inhibitor, verifying the reliability and applicability of this FUAS platform in variant fields.

SIGNIFICANCE

We demonstrate that by designing fluorogenic target competitors, FUAS can be alternatively developed in a label-free manner and with a higher sensitivity than the previously developed SSAS. This work opens a new way to develop high-performance aptamer-based sensors. Furthermore, our developed FUAS should inspire more interest for wide applications incluidng target-triggered drug deliveries when therapeutic fluorogenic target competitors are used.

Colorimetric-SERS dual-mode aptasensor for Staphylococcus aureus based on MnO2@AuNPs oxidase-like activity

The nanozyme-linked aptamer-sorbent assay (NLASA) is a rapid way to screen and characterize aptamer binding to targets. In this paper, a MnO2@AuNPs@aptamer (Apt) based NLASA coupled with colorimetric-SERS dual-mode for Staphylococcus aureus (S. aureus) detection is presented. Cu,Fe-CDs were used as the reducing agent to synthesize MnO2 and gold nanoparticles (AuNPs). Then, they were fabricated to obtain MnO2@AuNPs with oxidase (OXD)-like and SERS activities. The S. aureus aptamer was conjugated to MnO2@AuNPs and enhanced the OXD-like activity, which realized the specific capture of S. aureus in food matrices. In addition, S. aureus improves the oxidation of 2,2'-azino-bis (3-ethylbenzthiazoline-6-sulfonic acid (ABTS) but inhibits 3,3',5,5'-tetramethylbenzidine (TMB) to generate Raman-active oxTMB with MnO2@AuNPs@Apt. This sensor was used for detections of S. aureus in a concentration ranged from 101 to 107 CFU/mL with a detection limit of 0.926 CFU/mL (colorimetric) and 1.561 CFU/mL (SERS), and the recovery is 85%-105% in real samples.

Applications of nanomaterials with enzyme-like activity for the detection of phytochemicals and hazardous substances in plant samples

Plants such as herbs, vegetables, fruits, and cereals are closely related to human life. Developing effective testing methods to ensure their safety and quantify their active components are of significant importance. Recently, nanomaterials with enzyme-like activity (known as nanozymes) have been widely developed in various assays, including colorimetric, fluorescence, chemiluminescence, and electrochemical analysis. This review presents the latest advances in analyzing phytochemicals and hazardous substances in plant samples based on nanozymes, including some active ingredients, organophosphorus pesticides, heavy metal ions, and mycotoxins. Additionally, the current shortcomings and challenges of the actual sample analysis were discussed.

A new difunctional liquid crystal nanosurface molecularly imprinted polyitaconic acid nanoprobe for SERS/RRS determination of ultratrace melamine

In this paper, a new dimode scattering spectral method for rapid detection of ultratrace melamine (ML) in dairy products was established by coupling nanosurface molecular imprinting technology with nanocatalytic amplification reaction of liquid crystal particles. It was found that liquid crystal cholesteryl butyrate (CBU) nanosurface imprinted polymers (CBU@MIP) not only recognized ML but also catalyzed the nano indicator reaction of HAuCl4-sodium formate to produce gold nanoparticles with surface-enhanced Raman scattering (SERS) and resonance Rayleigh scattering (RRS) effect. When ML was added, it specifically combined with CBU@MIP to form CBU@MIP-ML conjugates with strong catalytic activity, and SERS and RRS signals increased linearly with the detection limits of 0.0072 pmol/L and 0.093 pmol/L respectively. The method was applied to the determination of ML in dairy products and plastic tablewares with relative standard deviation (RSD) of 2.2-4.4 % and 1.6-4.7 %, and recovery of 95.4 %-108.3 % and 95.9-108.6  % respectively.

Nanomaterials-based aptasensors: An efficient detection tool for heavy-metal and metalloid ions in environmental and biological samples

In light of potential risks of heavy metal exposure, diverse aptasensors have been developed through the combination of aptamers with nanomaterials for the timely and efficient detection of metals in environmental and biological matrices. Aptamer-based sensors can benefit from multiple merits such as heightened sensitivity, facile production, uncomplicated operation, exceptional specificity, enhanced stability, low immunogenicity, and cost-effectiveness. This review highlights the detection capabilities of nanomaterial-based aptasensors for heavy-metal and metalloid ions based on their performance in terms of the basic quality assurance parameters (e.g., limit of detection, linear dynamic range, and response time). Out of covered studies, dendrimer/CdTe@CdS QDs-based ECL aptasensor was found as the most sensitive option with an LOD of 2.0 aM (atto-molar: 10-18 M) detection for Hg2+. The existing challenges in the nanomaterial-based aptasensors and their scientific solutions are also discussed.

TbMOF@Au catalytic determination of trace malathion with aptamer SERS/RRS/Abs assay

Terbium metal-organic framework (TbMOF) was prepared by microwave method with 1,3,5-benzenetricarboxylic acid as ligand. With HAuCl₄ as precursor and NaBH₄ as reducing agent, TbMOF-loaded gold nanoparticles (AuNPs) catalyst (TbMOF@Au₁) was prepared rapidly and characterized by transmission electron microscope (TEM), X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. It was found that TbMOF@Au₁ has a strong catalytic effect on the HAuCl₄-Cys nanoreaction, and the produced AuNPs have a strong resonant Rayleigh scattering (RRS) peak and surface plasmon resonance absorption (Abs) peak at 370 nm and 550 nm, respectively. With the addition of the molecular probe Victoria blue 4R (VB4r), AuNPs have a strong surface-enhanced Raman scattering (SERS) effect, the target analyte molecules are trapped in between the nanoparticles and a hot spot effect created in the process resulting in an extremely high SERS signal. A new SERS/RRS/Abs triple-mode analysis method for Malathion (MAL) was established by coupling this new TbMOF@Au₁ catalytic indicator reaction with MAL aptamer (Apt) reaction, and its SERS detection limit was 0.21 ng/mL. The SERS quantitative analysis method has been applied to the analysis of fruit samples with the recovery of 92.6-106.6 % and the precision of 2.72-8.16 %.

Liquid Crystal@Nanosilver Catalytic Amplification-Aptamer Trimode Biosensor for Trace Pb2

Liquid crystals (LCs) are a very important display material. However, the use of LC, especially LC-loaded nanoparticles, as a catalyst to amplify the analytical signal and coupled with specific aptamer (Apt) as a recognition element to construct a highly sensitive and selective three-mode molecular spectral assay is rarely reported. In this article, five LCs, such as cholesteryl benzoate (CB), were studied by molecular spectroscopy to indicate the liquid crystal nanoparticles in the system, and highly catalytic and stable CB loaded-nanosilver (CB@AgNPs) sol was prepared. The slope procedure was used to study the catalysis of the five LCs and CB@AgNPs on the new indicator reaction between AgNO₃ and sodium formate (Fo) to produce silver nanoparticles (AgNPs) with a strong surface plasmon resonance absorption (Abs) peak at 450 nm, a resonance Rayleigh scattering (RRS) peak at 370 nm and a surface enhanced Raman scattering (SERS) peak at 1618 cm^-1 in the presence of molecular probes. By coupling the new CB@AgNPs catalytic indicator reaction with the Apt reaction, a new CB@AgNPs catalytic amplification-SERS/RRS/Abs trimode biosensoring platform was constructed for detecting inorganic pollutants, such as Pb²⁺, Cd²⁺, Hg²⁺ and As³⁺.

Utilizing inner filter effect in resonance Rayleigh scattering technique: a case study with silver nanocubes as RRS probe and several analytes as absorbers

It was demonstrated that the mechanism of the inner filter effect (IFE) can emerge well in the resonance Rayleigh scattering (RRS) technique and be utilized as a new analytical method in the design of innovative IFE-based sensors. To prove this process, silver nanocubes (Ag NCs) with tunable extinction spectra were selected as RRS probes, and three analytes, doxorubicin (DOX), sunitinib (SUN), and Alizarin Red S (ARS), were considered as the typical absorbers. In addition, in the presence of SUN as a typical analyte, the quenching of the RRS signal of Ag NCs, with λ_max of 419 nm, was linear in the range 0.01 to 2.5 µM of SUN. The limit of detection (LOD) was 0.0025 µM. The introduced method was then used to develop a dual-signal assay for the ratiometric determination of Al³⁺ ions. The suggested dual-signal assay was based on the color changes of ARS caused by Al³⁺ and the IFE between ARS and Ag NCs. The obtained results showed that the two characteristics of response sensitivity and linear dynamic range are very satisfactory for sensing Al³⁺ ions. The findings of this study demonstrate that the newly developed IFE mechanism can be employed as an attractive and highly efficient analytical technique for measuring different analytes.

A stable and sensitive Au metal organic frameworks resonance Rayleigh scattering nanoprobe for detection of SO3 2- in food based on fuchsin addition reaction

A stable Au metal organic frameworks (AuMOF) nanosol was prepared. It was characterized by electron microscopy and molecular spectral techniques. In pH 6.8 PBS buffer solution, AuMOF nanoprobes exhibit a strong resonance Rayleigh scattering (RRS) peak at 330 nm. After basic fuchsin (BF) adsorbing on the surface of AuMOF, the RRS energy of the nanoprobe donor can be transferred to BF receptor, resulting in a decrease in the RRS intensity at 330 nm. Both sulfite and BF taken place an addition reaction to form a colorless product (SBF) that exhibit weak RRS energy transfer (RRS-ET) between AuMOF and SBF, resulting in the enhancement of the RRS peak. As the concentration of SO3 2-increases, the RRS peak is linearly enhanced. Thus, a new and sensitive RRS-ET method for the detection of SO3 2- (0.160-5.00 μmol/L) was developed accordingly using AuMOF as nanoprobes, with a detection limit of 0.0800 μmol/L. This new RRS method was applied to determination of SO3 2- in food and SO2 in air samples. The recoveries of food and air samples were 97.1-106% and 92.9-106%, and the relative standard deviation (RSD) was 2.10-4.80% and 2.10-4.50%, respectively.

A novel liquid crystal resonance Rayleigh scattering spectral probe for determination of trace Cr6

Hexavalent chromium (Cr⁶⁺) has strong oxidizing property and toxicity. It has been identified as a carcinogen with obvious carcinogenic effect by the International Anti-cancer Research Center. Therefore, it has a great significance to establish a simple and sensitive method for Cr⁶⁺. In the solution, liquid crystal (LC) trans, trans-4-(3,4-difluorophenyl)-4'-n-pentylbicyclohexyl (DP) exhibits strong resonance Rayleigh scattering (RRS) effect due to formation DP nanoparticles. It was used firstly as nanoprobe, to establish a simple and sensitive RRS energy transfer (RRS-ET) method for the determination of trace Cr⁶⁺ in water samples. The Cr⁶⁺ reacts with diphenylcarbazide (DCB) to produce purple complex. It is adsorbed on the nanoprobe surface, the purple complex as energy receptor and DP as energy donor to produce RRS-ET phenomenon, to make the RRS signal of 370 nm decreasing. In the range of 3-30 nmol/L Cr⁶⁺, with the increase of concentration, the RRS signal decreased linearly at 370 nm, with a detection limit of 0.49 nmol/L. This new RRS-ET method was applied to the determination of Cr⁶⁺ in water samples, with recovery of 96.0-104.7% and the relative standard deviation (RSD) of 4.44-9.98%.

Highly catalysis amplification of MOFNd-loaded nanogold combined with specific aptamer SERS/RRS assay of trace glyphosate

A neodymium metal-organic framework (MOFNd) was prepared using 1H-pyrazole-3,5-dicarboxylic acid (H3pdc) and 2-pyrazinecarboxylic acid as ligands. Through the addition of HAuCl4 as a precursor and NaBH4 as a reducing agent, a new MOFNd-loaded nanogold (AuNPs) (Au@MOFNd) nanosol with good stability and high catalytic activity was conveniently prepared via a solvothermal-reduction method and characterized. It was found that the indicator reaction of reducing HAuCl4 by Na2SO3 to generate AuNPs was slow. Au@MOFNd strongly catalyzes this nanoreaction, and the produced AuNPs exhibit a strong resonance Rayleigh scattering (RRS) peak at 370 nm, and a strong surface-enhanced Raman scattering (SERS) peak at 1617 cm-1 with the addition of the molecular probe Victoria blue 4R (VB4r). A novel SERS/RRS di-mode quantitative analysis method for glyphosate (GLY) was established by coupling this new Au@MOFNd catalytic indicator reaction with the aptamer (Apt) reaction of GLY, with SERS and RRS detection limits of 0.02 nM and 0.3 nM, respectively. It has been applied to the analysis of soil samples with a recovery rate of 93.0%-106.5% and precision of 2.2%-4.1%, and the results were satisfactory.

PVP-coated silver nanocubes as RRS probe for sensitive determination of Haloperidol in real samples

Polyol synthesis of silver nanocubes (Ag NCs) under dark conditions yielded nanoparticles with high uniformity and purity, as well as edge lengths of 42 nm with good stability and scattering cross-section. These nanoparticles were characterized by SEM, TEM, and Uv-vis spectroscopy. The presence of polyvinylpyrrolidone (PVP) as a capping agent on the surface of Ag NCs, as well as its satisfactory interaction level with Haloperidol (Hp) as an antipsychotic drug, has led to the use of these nanoparticles as Resonance RayleighScattering (RRS) probe to measure Hp. Indeed, Hp resulted in reducing the RRS signal of Ag NCs, and this change in RRS intensity was linear in the range of 10.0 to 800.0 µg L^-1 of Hp. The limits of detection (LOD) and quantification (LOQ) were found to be 1.5 and 5.0 µg L^-1, respectively. The influence of interfering species was studied, and it was found that the suggested method has good selectivity and can be used to monitor Hp in actual samples. As a result, this RRS probe operated well in determining Hp in pharmaceutical and human plasma samples with satisfactory recovery.

A novel aptamer RRS assay platform for ultratrace melamine based on COF-loaded Pd nanocluster catalytic amplification

Two COFs of BzBD and BzBD loaded Pd nanoclusters (BzBD_Pd) were prepared using 1,3,5-benzenetricarboxaldehyde (Bz), benzidine (BD) and CO reducing agent, and were characterized by transmission electron microscopy (TEM), X-ray powder diffraction (XRD), infrared spectroscopy (IR) and other techniques. BzBD_Pd can strongly catalyze the new and stable Au@NiP nanoreaction that exhibit a strong resonance Rayleigh scattering (RRS) peak at 538 nm and a surface plasmon resonance (SPR) absorption peak at 395 nm, and the sensitive and facile RRS technique was used to study the indicator reaction. Combining the nanocatalytic amplification reaction with specific aptamer (Apt) of some target molecules such as melamine (ML), urea (UR) and bisphenol A (BPA), a simple, sensitive and selective Apt RRS assay platform was established. The linear range of the RRS detection platform for melamine is 0.0025-0.04 nmol/L, and the detection limit (DL) is 1.96 × 10^-4 nmol/L. In addition, ML in real sample was analyzed, the stability of BzBD, BzBD_Pd, PdNPs and the catalytic mechanism of COF_Pd were also considered.

Advances in Surface-Enhanced Raman Scattering-Based Aptasensors for Food Safety Detection

Owing to the excellent performances of high sensitivity, high specificity, on-site detection, and multiplexing capability, surface-enhanced Raman scattering (SERS)-based aptasensors have performed prosperous applications and gained impressive progress in food safety. Herein, we reviewed the SERS-based aptasensors from the principles to specific applications in food safety. First, the sensor-working principles, SERS label design and preparation are introduced. Then, the popular platforms in the aptasensors are summarized with their advantages and disadvantages, followed by their representative applications. Further, the specific applications of developing SERS-based aptasensors in food safety are systematically provided. Moreover, the multiplex analysis using SERS labels are highlighted. Finally, challenges and perspectives for improving the SERS-based aptasensor performance are also discussed, aiming to give some proposes for researchers to choose suitable SERS-based aptasensors according to specific applications.

A New Covalent Organic Framework of Dicyandiamide-Benzaldehyde Nanocatalytic Amplification SERS/RRS Aptamer Assay for Ultratrace Oxytetracycline with the Nanogold Indicator Reaction of Polyethylene Glycol 600

In this paper, dicyandiamide (Dd) and p-benzaldehyde (Bd) were heated at 180 °C for 3 h to prepare a new type of stable covalent organic framework (COF) DdBd nanosol with high catalysis. It was characterized by molecular spectroscopy and electron microscopy. The study found that DdBd had a strong catalytic effect on the new indicator reaction of polyethylene glycol 600 (PEG600)-chloroauric acid to form gold nanoparticles (AuNPs). AuNPs have strong resonance Rayleigh scattering (RRS) activity, and in the presence of Victoria Blue B (VBB) molecular probes, they also have a strong surface-enhanced Raman scattering (SERS) effect. Combined with a highly selective oxytetracycline (OTC) aptamer (Apt) reaction, new dual-mode scattering SERS/RRS methods were developed to quantitatively analyze ultratrace OTC. The linear range of RRS is 3.00 × 10-3 -6.00 × 10-2 nmol/L, the detection limit is 1.1 × 10-3 nmol/L, the linear range of SERS is 3.00 × 10-3-7.00 × 10-2 nmol/L, and the detection limit is 9.0 × 10-4 nmol/L. Using the SERS method to analyze OTC in soil samples, the relative standard deviation is 1.35-4.78%, and the recovery rate is 94.3-104.9%.

A Simple and Sensitive Nanogold RRS/Abs Dimode Sensor for Trace As3+ Based on Aptamer Controlled Nitrogen Doped Carbon Dot Catalytic Amplification

Using citric acid (CA) and ethylenediamine (EDA) as precursors, stable nitrogen-doped carbon dots (CD) nanosols were prepared by microwave procedure and characterized in detail. It was found that CD_Ns catalyze ethanol (Et)-HAuCl₄ to generate gold nanoparticles (AuNPs), which have strong surface plasmon resonance, Rayleigh scattering, (RRS) and a surface plasmon resonance (SPR) absorption (Abs) effect at 370 nm and 575 nm, respectively. Compled the new catalytic amplification indicator reaction with the specific As³⁺ aptamer reaction, a new RRS/Abs dual-mode aptamer sensor for the assay of trace As³⁺ was developed, based on the RRS/Abs signals increasing linearly with As³⁺ increasing in the ranges of 5-250 nmol/L and 50-250 nmol/L, whose detection limits were 0.8 nmol/L and 3.4 nmol/L As³⁺, respectively. This analytical method has the advantages of high selectivity, simplicity, and rapidity, and it has been successfully applied to the detection of practical samples.

Sensitive Aptamer SERS and RRS Assays for Trace Oxytetracycline Based on the Catalytic Amplification of CuNCs

A new method for the determination of oxytetracycline (OTC) has been established by coupling the catalytic amplification reaction of copper nanoclusters (CuNCs) with the aptamer reaction. CuNCs prepared by a wet chemical method have the catalytic activity for the formation of gold nanoparticles (AuNPs) resulting from a HAuCl₄-ethanol (En) reaction. The experimental results showed that OTC aptamer (Apt) can be adsorbed on the surface of CuNCs in a non-specific way, thus inhibiting its catalytic activity. When OTC was added to the solution, the OTC-Apt complex was generated by a specific reaction, which made the CuNCs desorb and restore their catalytic activity. With the increase of OTC, the recovery of the catalytic activity of CuNCs is strengthened, the reaction speed is accelerated, and the number of AuNPs is increased. The generated AuNPs exhibited surface enhanced Raman scattering (SERS) signals at 1615 cm⁻¹ in the presence of Vitoria blue 4R (VB4R) molecular probes, and a resonance Rayleigh scattering (RRS) peak at 586 nm. There is a good linear relationship between the intensities of SERS, or RRS, and OTC concentration at the range of 37.5–300 ng/L or 37.5–225 ng/L, respectively. A new SERS and RRS assay for the determination of trace OTC based on the regulation of CuNCs catalysis was established.

A novel small molecular liquid crystal catalytic amplification-nanogold SPR aptamer absorption assay for trace oxytetracycline

Liquid crystals (LCs) have been applied for a long time in the field of analytical chemistry. To date, there are no reports about utilization of LCs as the catalyst to amplification analytical signal. In this article, three small molecules LCs in water aqueous solutions were characterized using molecular spectra and particle size analysis. The characterization indicated that there are nanoparticles in the system. Among the them, 4-heptylbenzoic acid (HPB) exhibits the most sensitive performance in the analytical system based on the reduction of HAuCl₄ to gold nanoparticles (AuNPs) by NaH₂PO₂ by the spectrophotometric slope evaluation procedure. As the concentration of LCs catalyst increases, the AuNPs surface plasmon resonance (SPR) absorption peak at 550 nm increases linearly, that can be utilized to amply the absorption signal. Based on the LCs catalytic amplification reaction and immunoreaction, a new SPR spectrophotometric analysis method was developed for the label-free detection of oxytetracycline, with a detection limit of 0.50 ng/mL. The method was also successfully applied for the detection of oxytetracycline-spiked environmental water samples to demonstrate its practical usefulness.

Single-atom Fe catalytic amplification-gold nanosol SERS/RRS aptamer as platform for the quantification of trace pollutants

Bisphenol A (BPA), as a typical endocrine disruptor, poses a serious threat to human health. Therefore, it is urgent to establish a rapid, sensitive, and simple method for the determination of BPA. In this paper, based on the aptamer-mediated single-atom Fe carbon dot catalyst (SA_Fe) catalyzing the HAuCl₄-ethylene glycol (EG) nanoreaction, a new SERS/RRS di-mode detection method for BPA was established. The results show that SA_Fe exhibits a strong catalytic effect on the HAuCl₄-EG nanoreaction, which could generate purple gold nanoparticles (AuNPs) with resonance Rayleigh scattering (RRS) signals and surface-enhanced Raman scattering (SERS) effects. After the addition of BPA aptamer (Apt), it could encapsulate SA_Fe through intermolecular interaction, thus inhibiting its catalytic action, resulting in the reduction of AuNPs generated and the decrease of RRS and SERS signals of the system. With the addition of BPA, Apt was specifically combined with BPA, and SA_Fe was re-released to restore the catalytic ability; the generated AuNPs increased. As a result of this RRS and SERS signals of the system recovered, and their increment was linear with the concentration of BPA. Thus, the quantification of 0.1-4.0 nM (RRS) and 0.1-12.0 nM (SERS) BPA was realized, and the detection limits were 0.08 nM and 0.03 nM, respectively. At the same time, we used molecular spectroscopy and electron microscopy to study the SA_Fe-HAuCl₄-ethylene glycol indicator reaction, and proposed a reasonable SA_Fe catalytic reaction mechanism. Based on Apt-mediated SA_Fe catalysis gold nanoreaction amplification, a SERS/RRS di-mode analytical platform was established for targets such as BPA.