Target catalyzed hairpin assembly for constructing a ratiometric electrochemical aptasensor

Emerging insights into the application of metal-organic framework (MOF)-based materials for electrochemical heavy metal ion detection

Heavy metal ions are one of the main sources of water pollution, which has become a major global problem. Given the growing need for heavy metal ion detection, electrochemical sensor stands out for its high sensitivity and efficiency. Metal-organic frameworks (MOFs) have garnered much interest as electrode modifiers for electrochemical detection of heavy metal ions owing to their significant specific surface area, tailored pore size, and catalytic activity. This review summarizes the progress of MOF-based materials, including pristine MOFs and MOF composites, in the electrochemical detection of various heavy metal ions. The synthetic methods of pristine MOFs, the detection mechanisms of heavy metal ions and the modification strategies of MOFs are introduced. Besides, the diverse applications of MOF-based materials in detecting both single and multiple heavy metal ions are presented. Furthermore, we present the current challenges and prospects for MOF-based materials in electrochemical heavy metal ion detection.

Ultrasensitive rapid detection of antibiotic resistance genes by electrochemical ratiometric genosensor based on 2D monolayer Ti3C2@AuNPs

As an important emerging pollutant, antibiotic resistance genes (ARGs) monitoring is crucial to protect the ecological environment and public health, but its rapid and accurate detection is still a major challenge. In this study, a new single-labeled dual-signal output ratiometric electrochemical genosensor (E-DNA) was developed for the rapid and highly sensitive detection of ARGs using a synergistic signal amplification strategy of T3C2@Au nanoparticles (T3C2@AuNPs) and isothermal strand displacement polymerase reaction (ISDPR). Specially, two-dimensional monolayer T3C2 nanosheets loaded with uniformly gold nanoparticles were prepared and used as the sensing platform of the E-DNA sensor. Benefiting from excellent conductivity and large specific surface area of Ti3C2@AuNPs, the probe immobilization capacity of the E-DNA sensor is doubled, and electrochemical response signals of the E-DNA sensor were significantly improved. The proposed single-labeled dual-signal output ratiometric sensing strategy exhibits three to six times higher sensitivity for the sul2 gene than the single-signal sensing strategy, which significantly reduces cost meanwhile retaining the advantages of high sensitivity and reliability offered by conventional dual-labeled ratiometric sensors. Coupled with ISDPR amplification technology, the E-DNA sensor has a wider linear range from 10 fM to 10 nM and a limit of detection as low as 2.04 fM (S/N=3). More importantly, the E-DNA sensor demonstrates excellent specificity, good stability and reproducibility for target ARGs detection in real water samples. The proposed new sensing strategy provides a highly sensitive and versatile tool for the rapid and accurate quantitative analysis of various ARGs in environmental water samples.

Electrochemical aptasensor based on Ce3NbO7/CeO2@Au hollow nanospheres by using Nb.BbvCI-triggered and bipedal DNA walker amplification strategy for zearalenone detection

Herein, an electrochemical aptasensor combining Nb.BbvCI-triggered bipedal DNA walking strategy was constructed for ultrasensitive assay of zearalenone (ZEN). The aptasensor used Ce₃NbO₇/CeO₂ @Au hollow nanospheres as electrode modification material and PdNi@MnO₂/MB as the signal label. Importantly, the Ce₃NbO₇/CeO₂ synthesized by hydrothermal method were combined with Au nanoparticles and applied to the electrode surface. The as-prepared Ce₃NbO₇/CeO₂ @Au possessed a large surface area, excellent electrical conductivity, stability and more binding sites. PdNi@MnO₂ with high specific surface area and porosity combined with molecule methylene blue (MB) was introduced into electrodes as the signal label. The proposed aptasensor utilized the advantages of specific recognition of aptamers and target molecules to release bipedal DNA walker (w-DNA), and then the w-DNA was triggered by Nb.BbvCI and entered the cycle to release more signal probes. The feasibility of this strategy was recorded by the differential pulse voltammetry (DPV) method. Under the optimized conditions, the electrochemical aptasensor exhibited a wide linear dynamic range from 1 × 10^-4 to 1 × 10³ ng mL^-1 with a low detection limit of 4.57 × 10^-6 ng mL^-1. Moreover, the aptasensor had high selectivity, good stability, excellent repeatability and provided an effective method for the trace detection of ZEN in real samples.

Development of Enzyme-Free DNA Amplifier Based on Chain Reaction Principle

Enzyme-free DNA amplifiers can amplify the signal of nucleic acid molecules. They can be applied to DNA molecular operation and nucleic acid detection. The reaction speed is the core index to evaluate DNA amplifiers. In this study, we designed a DNA amplifier based on an enzyme-free chain reaction. This DNA amplifier can release one more signal molecule in each round of reaction and trigger the next round, which significantly improved reaction speed. Moreover, because the amplifier used a stable DNA structure, the reaction can occur at room temperature. To integrate the amplifier into other DNA molecular operations, we performed the amplification reaction in a microfluidic chip module. The results showed that the amplifier can realize real-time signal feedback at a proper input molecule concentration and reach the endpoint in 40 s, even at a low relative concentration. To apply the amplifier for nucleic acid detection, we also used a conventional fluorescent polymerase chain reaction instrument for the reaction. The results showed that the amplifier specifically detected trace DNA single-stranded molecules. To solve the leakage problem of existing amplifiers, we designed a DNA molecule as the chain reaction's inhibitor, which was crucial in controlling the reaction speed and preventing leakage. STATEMENT OF SIGNIFICANCE: Traditional amplifier strategies of enzyme-free DNA amplifiers relied on a constant number of cycling molecules to catalyze the amplifier molecules' changing structure and release fluorescent signals, which lead low reaction speed. Based on an enzyme-free chain reaction, we designed a DNA amplifier which can release one more cycling molecule in each loop and trigger the next loop and significantly improve reaction speed in this study. Our analysis on microfluidic chip module and PCR instrument verifies high sensitivity and selectivity. And this strategy of DNA amplifier realizes the control of reaction and prevents leakage. We believe that this automated amplification strategy could have great applications in vivo signal detection, imaging, and signal molecule translation.

Reagentless detection of staphylococcal enterotoxin B via electrochemical interrogation of conformational changes

An electrochemical biosensor for staphylococcal enterotoxin B (SEB) detection has been designed on the basis of electrochemical interrogation of conformational changes. Ferrocene-labeled hairpin probe (Fc-HP) and SEB aptamer are introduced for the construction of the platform. Without SEB, the rigid construction of DNA duplex that included SEB aptamer and Fc-HP prevented Fc getting access to the electrode surface, keeping the "eT-off" state in the detection system. In the presence of SEB, the interaction between SEB and the aptamer could trigger the disruption of DNA duplex and the restoration of hairpin structure, accompanied by the increase of Fc oxidation current. The decreasing distance between the redox probe and electrode upon the nucleic acid reconfiguration substantially increased the efficiency of eT, which resulted in the enhanced Fc signal. The proposed strategy presented a wide linear detection range from 0.005 to 100 ng mL^-1 with a detection limit down to 3 pg mL^-1 (S/N = 3). To investigate the applicability and reliability of the method in real food samples such as milk samples, we compared the results between this method and the commercial ELISA kit. The relative percentage error between the two assays ranged from -6.42% to 6.31%, indicating that there was no obvious difference between the results.

A label-free and modification-free ratiometric electrochemical strategy for enhanced natural enzyme detection using a bare electrode and nanozymes system

Ratiometric electrochemical assays have been demonstrated to be more sensitive and selective in various sensing events, mainly due to their affordable built-in correction and good self-reference capability. But it is known that complicated modification and labeling operations usually are necessary for the construction of ratiometric electrochemical assays, therefore is a hot and important issue needing consideration carefully. We herein report a new yet simple bare electrode-based ratiometric electrochemical bioassay to achieve sensitive and selective analysis of alkaline phosphatase (ALP), using a liquid phase system that contains CoOOH nanozymes and commercially available indicator substrate. This proposed bioassay works based on the ratiometric change of dual electrochemical signals, arising from an exclusive target ALP-triggered hydrolysis of electrochemical substrate p-nitrophenyl phosphate (PNPP). In this design, the two hydrolyzed products of electrochemically active p-nitrophenol (PNP) and electrochemically inactive phosphate anion (PO₄^3-) are responsible together for the ratiometric electrochemical analysis of ALP. PNP exhibits a straightforward current response toward ALP content; however, PO₄^3- cannot show a direct electrochemical signal thus is rationally designed to offer an alternative response by linking it with the specific CoOOH nanozyme-catalyzed reaction of 3,3',5,5'-tetramethylbenzidine (TMB) and H₂O₂, in which the nanozyme-catalyzed product oxTMB shows a direct reduction current at the GCE, and significantly decreases with increasing PO₄^3- species due to the good inhibition of PO₄^3- toward CoOOH nanozyme activity. As a result, a ratiometric electrochemical strategy for ALP analysis with a low limit of detection of 0.366 U/L (S/N = 3) was successfully achieved by integrating the above direct and indirect dual electrochemical responses. This developed bioassay can allow the quantitative diagnosis of ALP activity especially with a label-free and modification-free merit, therefore paving the way for simple, convenient, and portable electroanalytical tools in biosensing design and application.

A novel ratiometric electrochemical biosensing strategy based on T7 exonuclease-assisted homogenous target recycling coupling hairpin assembly-triggered double-signal output for the multiple amplified detection of miRNA

A novel ratiometric electrochemical biosensing strategy based on T7 exonuclease (T7 Exo)-assisted homogenous target recycling coupling hairpin assembly triggered dual-signal output was proposed for the accurate and sensitive detection of microRNA-141 (miRNA-141). Concretely, in the presence of target miRNA, abundant signal transduction probes were released via the T7 Exo-assisted homogenous target recycling amplification, which could be captured by the specially designed ferrocene-labeled hairpin probe (Fc-H1) on -electrode interface and triggered the nonenzymatic catalytic hairpin assembly (Fc-H1 + MB-H2) to realize the cascade signal amplification and dual-signal output. Through such a conformational change process, the electrochemical signal of Fc (IFc) and MB (IMB) is proportionally and substantially decreased and increased. Therefore, the signal ratio of IMB/IFc can be employed to accurately reflect the true level of original miRNA. Benefiting from the efficient integration of the T7 Exo-assisted target recycle, nonenzymatic hairpin assembly and dual-signal output mode, the proposed sensor could realize the amplified detection of miRNA-141 effectively with a wide detection range from 1 fM to 100 pM, and a detection limit of 200 aM. Furthermore, it exhibits outstanding sequence specificity to discriminate mismatched RNA, acceptable reproducibility and feasibility for real sample. This strategy effectively integrated the advantages of multiple amplification and ratiometric output modes, which could provide an accurate and efficient method in biosensing and clinical diagnosis.

Ratiometric Electrochemistry: Improving the Robustness, Reproducibility and Reliability of Biosensors

Electrochemical biosensors are an increasingly attractive option for the development of a novel analyte detection method, especially when integration within a point-of-use device is the overall objective. In this context, accuracy and sensitivity are not compromised when working with opaque samples as the electrical readout signal can be directly read by a device without the need for any signal transduction. However, electrochemical detection can be susceptible to substantial signal drift and increased signal error. This is most apparent when analysing complex mixtures and when using small, single-use, screen-printed electrodes. Over recent years, analytical scientists have taken inspiration from self-referencing ratiometric fluorescence methods to counteract these problems and have begun to develop ratiometric electrochemical protocols to improve sensor accuracy and reliability. This review will provide coverage of key developments in ratiometric electrochemical (bio)sensors, highlighting innovative assay design, and the experiments performed that challenge assay robustness and reliability.

Application of electrochemical methods for the detection of abiotic stress biomarkers in plants

Abiotic stress is the main cause of low productivity in plants. Therefore, it is important to detect stress and respond to it in a timely manner to avoid irreversible damage to plant productivity and health. The application of traditional methods in agriculture is limited by expensive equipment and cumbersome sample processing. More effective detection methods are urgently needed due to the trace amounts and low stabilities of plant biomarkers. Electrochemical detection methods have the unique advantages of high accuracy, a low detection limit, fast response and easy integration with systems. In this review, the application of three types of electrochemical methods to phytohormone assessment is highlighted including direct electrochemical, immunoelectrochemical, and photoelectrochemical methods. Research on electrochemical methods for detecting abiotic stress biomarkers, including various phytohormones, is also summarized with examples. To date, the detection limit of exogenous plant hormones can reach pg/mL or even lower. Nevertheless, more efforts need to be made to develop a portable instrument for in situ online detection if electrochemical sensors are to be applied to the detection of the endogenous hormones or the physiological state of plants. Additionally, plant-wearable sensors that can be directly attached to or implanted into plants for continuous, noninvasive and real-time monitoring are emphasized. Finally, rational summaries of the considered methods and present challenges and future prospects in the field of abiotic stress detection-based electrochemical biosensors are thoroughly discussed.

Advances of aptamers screened by Cell-SELEX in selection procedure, cancer diagnostics and therapeutics

Aptamers are a class of short artificial single-stranded oligo(deoxy) nucleotides that can bind to different targets, which generated by Systematic Evolution of Ligands by Exponential Enrichment (SELEX). Due to excellent selectivity and high affinity to targets, aptamers hold considerable potential as molecular probe in diverse applications ranging from ensuring food safety, monitoring environment, disease diagnosis to therapy. This review highlights recent development and challenges about aptamers screened by Cell-SELEX, and its application about cancer diagnostics and therapeutics. Advances about some operation methods such as seperation method and culture method in aptamers selection procedure were summarized in this paper. Some common challenges and technological difficulties such as nonspecific binding and biostability were discussed. Up to now, the recent endeavors about cancer diagnostic and therapeutic applications of aptamers are summarized and expatiated. Most of aptamers screened by Cell-SELEX took tumor cells as target cells, and such aptamers have been assembled to various aptasensor for cancer diagnosis. Aptamers conjugated various drugs or nanomaterials are functioned for cancer target therapy to improve drugs delivery efficiency and reduce side effects. Furthermore, the duplexed aptamer is discussed to be applied for cancer cells detection and some conflicts of theories about duplexed aptamer designs are analyzed.

Highly sensitive and specific electrochemical biosensor for microRNA-21 detection by coupling catalytic hairpin assembly with rolling circle amplification

BACKGROUND

MicroRNA plays a significant role in gene regulation and is usually regarded as an important biological marker. Electrochemical biosensors are excellent tools for microRNA detection.

METHODS

In this experiment, we take miRNA-21 as a target, combining catalytic hairpin assembly (CHA) and rolling circle amplification (RCA) as a dual signal amplification strategy for the detection of microRNA in an electrochemical biosensor.

RESULTS

This strategy has a good linear range of 0.5-12 500 pmol of microRNA. The limit of detection (LOD) for miRNA is as low as 290 fmol, showing excellent performance. Finally, this method has been successfully applied to the detection of miRNA-21 from HeLa cells.

CONCLUSION

This method can be applied not only for microRNA detection with high sensitivity and speed, but can also detect small molecules and proteins combined with aptamers.

A versatile label-free electrochemical biosensor for circulating tumor DNA based on dual enzyme assisted multiple amplification strategy

A versatile label-free electrochemical biosensor based on dual enzyme assisted multiple amplification strategy was developed for ultrasensitive detection of circulating tumor DNA (ctDNA). The biosensor consists of a triple-helix molecular switch (THMS) as molecular recognition and signal transduction probe, ribonuclease HII (RNase HII) and terminal deoxynucleotidyl transferase (TdT) as dual enzyme assisted multiple amplification accelerator. The presence of target ctDNA could open THMS and trigger RNase HII-assisted homogenous target recycling amplification to produce substantial signal transduction probe (STP). The released STP hybridized with the capture probe immobilized on a gold electrode, then TdT and assistant probe were further employed to fulfill TdT-mediated cascade extension and generate stable DNA dendritic nanostructures. The electroactive methyl blue (MB) was finally used as the signal reporter to realize the multiple electrochemical amplification ctDNA detection as the amount of MB is positively correlated with the target ctDNA. Combined with the efficient recognition capacity of the designed THMS and the excellent multiple amplification ability of RNase HII and TdT, the constructed sensing platform could detect KRAS G12DM with a wide detection range from 0.01 fM to 1 pM, and the limit of detection as low as 2.4 aM. Besides, the platform is capable of detecting ctDNA in biological fluid such as plasma. More importantly, by substituting the loop of THMS with different sequences, this strategy could be conveniently expanded into the detection of other ctDNA, showing promising potential applications in clinical cancer screening and prognosis.

Ultrasensitive and label-free electrochemical aptasensor of kanamycin coupling with hybridization chain reaction and strand-displacement amplification

This work reports the proof-of-concept of an ultrasensitive label-free electrochemical aptasensor for Kanamycin (Kana) detection coupling strand-displacement amplification (SDA) with hybridization chain reaction (HCR). In the presence of target Kana, the analyte triggers conformational change of hairpin HP1 (HP1) and two-staged SDA to produce short single-stranded DNA (S1) with the aid of KF polymerase and nicking endonuclease. Meanwhile, the as-produced S1 hybridizes with the immobilized hairpin HP2 (HP2) on the electrode to open the hairpin, thereby resulting in the formation of DNA duplex. Thereafter, DNA duplex is selectively digested by Exo III accompanying S1 recycling. The residual single-stranded probe (S2) on the electrode opens another two hairpins in sequence and propagates a chain reaction of hybridization events between two alternating hairpins (H1 and H2) to form a long nicked double-helix. Upon addition of redox-active methylene blue (MB), numerous indicators are intercalated into the grooves of double-helix DNA polymers, each of which produces an electrochemical signal within the applied potentials. Under optimum conditions, the SDA/HCR-based electrochemical aptasensor exhibits a high sensitivity for detection of Kana down to 36 fM with a linear range from 0.05 to 200 pM. Additionally, the as-prepared aptasensor is successfully employed to determinate the Kana in animal derived food (milk). With the advantages of high sensitivity, label-free strategy and excellent selectivity, the developed aptasensor possesses great potential application value in food-safety analysis field.

Magnetic force assisted electrochemical sensor for the detection of thrombin with aptamer-antibody sandwich formation

A magnetic force assisted electrochemical aptamer-antibody sandwich assay (MESA) was developed for the detection of thrombin as a model protein in serum samples. The MESA using the formation of sandwich complexes on the electrochemical sensor probe for reaction and the removal of unbound bioconjugates from the sensor surface without washing are controlled by a magnetic field. Thrombin was determined by the cathodic currents of a toluidine blue O (TBO) attached with thrombin antibody modified magnetic nanoparticle (MNP) at the sensor surface. To detect thrombin in a serum sample, we applied a thrombin-specific aptamer as the capture molecule bound to the functionalized conducting polymer layer (poly-(2,2´:5´,5″-terthiophene-3´-p-benzoic acid) (pTBA)), and streptavidin and starch coated-MNP was conjugated with biotinylated thrombin antibodies (Ab) and TBO as the bioconjugate (MNP@Ab-TBO). The characterization of MNP@Ab-TBO and sensor probe was performed using voltammetry, impedance spectroscopy, XPS, and UV-VIS spectroscopy. The experimental conditions were optimized in terms of pH, binding time, removal time of unbound bioconjugates, and applied potential. The dynamic ranges of thrombin were from 1.0 to 500 nM with detection limit of 0.49 ( ± 0.06) nM. The recovery test demonstrates the reliability of the proposed sensing system for a handheld device.

Universal ratiometric electrochemical biosensing platform based on mesoporous platinum nanocomposite and nicking endonuclease assisted DNA walking strategy

The occurrence and development of many complex diseases are associated with various molecules, whose contents are rarely in the early stage of the disease. Thus a universal platform for the ultrasensitive detection of multilevel biomarkers should be developed. In this study, we introduced an electrochemical biosensing system based on nicking endonuclease (Nt.BbvCI) assisted DNA walking strategy. We successfully constructed a universal signal-off-on ratiometric electrochemical biosensor for various biomolecules, including small molecules, nucleic acids, and proteins, by progressively optimizing the schematics (schemes 1, 2, and 3). The MB-hairpin probes (MB-HPs) acted as a signal-off probe, and nanocomposites (MPNs@DOX@DNA2) acted as a conventional signal-on probe (scheme 3). With the aid of the MPNs@DOX@DNA2 and Nt.BbvCI assisted DNA walking mechanism, the designed ratiometric electrochemical biosensor showed a high sensitivity and broad detection range. In addition, the proposed method can be utilized to detect diverse targets quantitatively by changing the sequence of aptamers under optimum experimental conditions. Furthermore, it has been widely proved to realize well-accepted signal response in identifying complex samples, thereby resulting in an wide prospect for bioanalysis and clinical diagnosis.

Ratiometric electrochemical detection of hydrogen peroxide and glucose

Hydrogen peroxide (H₂O₂) detection is of high importance as it is a versatile (bio)marker whose detection can indicate the presence of explosives, enzyme activity and cell signalling pathways. Herein, we demonstrate the rapid and accurate ratiometric electrochemical detection of H₂O₂ using disposable screen-printed electrodes through a reaction-based indicator assay. Ferrocene derivatives equipped with self-immolative linkers and boronic acid ester moieties were synthesised and tested, and, through a thorough assay optimisation, the optimum probe showed good stability, sensitivity and selectivity towards H₂O₂. The optimised conditions were then applied to the indirect detection of glucose via an enzymatic assay, capable of distinguishing 10 μM from the background within minutes.

A novel "signal-on/off" sensing platform for selective detection of thrombin based on target-induced ratiometric electrochemical biosensing and bio-bar-coded nanoprobe amplification strategy

A novel dual-signal ratiometric electrochemical aptasensor for highly sensitive and selective detection of thrombin has been designed on the basis of signal-on and signal-off strategy. Ferrocene labeled hairpin probe (Fc-HP), thrombin aptamer and methyl blue labeled bio-bar-coded AuNPs (MB-P3-AuNPs) were rationally introduced for the construction of the assay platform, which combined the advantages of the recognition of aptamer, the amplification of bio-bar-coded nanoprobe, and the ratiometric signaling readout. In the presence of thrombin, the interaction between thrombin and the aptamer leads to the departure of MB-P3-AuNPs from the sensing interface, and the conformation of the single stranded Fc-HP to a hairpin structure to take the Fc confined near the electrode surface. Such conformational changes resulted in the oxidation current of Fc increased and that of MB decreased. Therefore, the recognition event of the target can be dual-signal ratiometric electrochemical readout in both the "signal-off" of MB and the "signal-on" of Fc. The proposed strategy showed a wide linear detection range from 0.003 to 30nM with a detection limit of 1.1 pM. Moreover, it exhibits good performance of excellent selectivity, good stability, and acceptable fabrication reproducibility. By changing the recognition probe, this protocol could be easily expanded into the detection of other targets, showing promising potential applications in disease diagnostics and bioanalysis.

Conformational Changes of Enzymes and Aptamers Immobilized on Electrodes

Conformation constitutes a vital property of biomolecules, especially in the cases of enzymes and aptamers, and is essential in defining their molecular recognition ability. When biomolecules are immobilized on electrode surfaces, it is very important to have a control on all the possible conformational changes that may occur, either upon the recognition of their targets or by undesired alterations. Both enzymes and aptamers immobilized on electrodes are susceptible to conformational changes as a response to the nature of the charge of the surface and of the surrounding environment (pH, temperature, ionic strength, etc.). The main goal of this review is to analyze how the conformational changes of enzymes and aptamers immobilized on electrode surfaces have been treated in reports on biosensors and biofuel cells. This topic was selected due to insufficient information found on the actual conformational changes involved in the function of these bioelectrochemical devices despite its importance.

A new electrochemical aptasensor based on a dual-signaling strategy and supersandwich assay

In this study, we develop a new electrochemical aptasensor by coupling two amplification strategies, including a dual signaling strategy and a supersandwich assay. In order to fabricate this aptasensor, a thiolated capture probe (CP) was first self-assembled on the gold electrode surface by Au-S bonds. After the addition of methylene blue (MB) modified signal probe 1 (SP1) and ferrocene (Fc) labeled signal probe 2 (SP2), supersandwich structure DNA, including multiple units of SP1 and SP2, was grown from the CP on the electrode surface. In the presence of ATP, the strong interaction between ATP and its aptamer (CP, SP1) leads to the disassembly of the supersandwich structure and thereby, the release of SP1 and SP2 from the gold electrode surface, resulting in a decrease of the MB and Fc signals. Taking "Signal gainMB + Signal gainFc" as the response signal, ATP can be detected sensitively; the detection limit is 2.1 nM, which is lower than that using either a single-signaling strategy or a traditional sandwich assay alone. Moreover, the new aptasensor also exhibits excellent specificity, selectivity, reliability and applicability. We believe that this new strategy will be helpful for fabricating sensitive and selective electrochemical aptasensors of other biomolecules and small molecules.

Dual-primer self-generation SERS signal amplification assay for PDGF-BB using label-free aptamer

Highly sensitive detection of proteins, especially those associated with cancers, is essential to biomedical research as well as clinical diagnosis. In this work, a simple and novel one-two-three signal amplification surface-enhanced Raman scattering (SERS) method for the detection of protein is fabricated by using label-free aptamer and dual-primer self-generation. Platelet-derived growth factor B-chain (PDGF-BB) is selected as the model protein. The one-two-three cascade DNA amplification means one target-aptamer binding event, two hairpin DNA switches and three DNA amplification reactions. This strategy possesses some remarkable features compared to conventional signal amplification methods: (i) A smart probe including a label-free aptamer is fabricated, for suitable hybridization without hindering the affinity of the aptamer toward its target. (ii) Using the unique structure switch of the aptamer and cooperator, a one-two-three working mode is developed to amplify the SERS signal. The amplification efficiency is enhanced. Given the unique and attractive characteristics, a simple and universal strategy is designed to accomplish ultrasensitive detection of proteins. The detection limit of PDGF-BB via SERS detection is 0.42 pM, with the linear range from 1.0×10(-12)M to 10(-8)M. It is potentially universal because the aptamer can be easily designed for biomolecules whose aptamers undergo similar conformational changes.