A Label-Free Colorimetric Aptasensor Containing DNA Triplex Molecular Switch and AuNP Nanozyme for Highly Sensitive Detection of Saxitoxin

QI Xiaoyan, LI Ling, YAN Xiaochen, ZHAO Yinglin, WANG Lele, MA Rui,WANG Sai,, and MAO Xiangzhao, 2)

A Label-Free Colorimetric Aptasensor Containing DNA Triplex Molecular Switch and AuNP Nanozyme for Highly Sensitive Detection of Saxitoxin

QI Xiaoyan1), LI Ling1), YAN Xiaochen1), ZHAO Yinglin1), WANG Lele1), MA Rui1),WANG Sai1),*, and MAO Xiangzhao1), 2)

1),,266003,2),266237,

Saxitoxin (STX), one of the most toxic paralytic shellfish poisons discovered to date, is listed as a required item of aqua- tic product safety inspection worldwide. However, conventional detection methods for STX are limited by various issues, such as low sensitivity, complicated operations, and ethical considerations. In this study, an aptamer-triplex molecular switch (APT-TMS) and gold nanoparticle (AuNP) nanozyme were combined to develop a label-free colorimetric aptasensor for the rapid and highly sensitive de- tection of STX. An anti-STX aptamer designed with pyrimidine arms and a purine chain was fabricated to form an APT-TMS. Spe- cific binding between the aptamer and STX triggered the opening of the switch, which causes the purine chains to adsorb onto the surface of the AuNPs and enhances the peroxidase-like activity of the AuNP nanozyme toward 3,3’,5,5’-tetramethylbenzidine. Under optimized conditions, the proposed aptasensor showed high sensitivity and selectivity for STX, with a limit of detection of 335.6pmolL?1and a linear range of 0.59–150nmolL?1. Moreover, good recoveries of 82.70%–92.66% for shellfish and 88.97%–106.5% for seawater were obtained. The analysis could be completed within 1h. The proposed design also offers a robust strategy to achieve detection of other marine toxin targets by altering the corresponding aptamers.

saxitoxin; colorimetric aptasensor; aptamer; triplex molecular switch; AuNP nanozyme

1 Introduction

Shellfish toxins are toxic metabolites produced by harm- ful algae. These toxins can be easily accumulated in the aquatic food chain, including seawater, shellfish, and fish, and can cause serious pollution and health risks. Accord- ing to their different symptoms and mechanisms of action, shellfish toxins can be classified as paralytic shellfish poi- sons (PSP), diarrheic shellfish poisons, amnesic shellfishpoisons, neurotoxic shellfish poisons, and azaspiracid shell- fish poisons (Oshima., 1987; Steidinger, 1993). Saxi- toxin (STX) is characterized by extreme toxicity, and isone of the most toxic PSP biotoxins discovered to date. Its toxicity is 1000 times that of sodium cyanide and 80 timesthat of cobra venom. STX can cause paralytic shellfish poi- soning and even death in severe cases (Oshima., 1987; Steidinger, 1993; Wiese., 2010). The median lethal dose (LD50) of STX to mouses is 10μgkg?1. Given its to- xic properties, STX has been listed as a required item of aquatic product safety inspection worldwide (Chang, 1997). The World Health Organization stipulates that the concentration of STX in edible parts of shellfish shouldnot exceed 80μg per 100g (Fraga, 2012). Several me- thods, such as mouse bioassay (Park, 1986), high-performance liquid chromatography(Bates, 1975; Turner, 2009), and enzyme-linked immunoassay (Li, 2020), are employed to detect STX. However, these methods are limited by a number of issues, such as time- consuming procedures, high costs, complicated operations, and ethical considerations(Bates, 1975; Park, 1986; Turner, 2009; Li, 2020). Therefore, the development of simple and rapid methods for the highly sensitive detection of STX is very important.

Aptamers are oligonucleotide molecules and are gene- rated byscreening through Systematic Evolution ofLigands by EXponential enrichment (SELEX). These mole- cules show high affinity and specificity toward their tar- gets (Nutiu, 2005), which include toxins (Moez, 2019), pathogenic bacteria(Abdelrasoul, 2020), an- tibiotics (Zhang, 2020), and heavy metals (Tian, 2017). The anti-STX aptamer was selected by Handy. (2013) and optimized to achieve higher affinity by Zheng(2015); these studies lay an excellent foundation for the construction of novel aptasensors for STX detection (Wang, 2020).

Colorimetric aptasensors represent a new research hot- spot and show great potential in visual and high-through- put detection. AuNPs are promising nanomaterials widely used as convenient transducers to construct various colo- rimetric aptasensors on account of their size-dependent sur-face plasmon resonance properties and specific surface area(Storhoff, 1998; Lee., 2008). The peroxidase- like catalytic ability of these materials enables their appli- cation as super nanozymes in biosensing assays (Gao, 2007; Yang, 2019). For instance, Yang(2020)developed a colorimetric aptasensor based on the enhancedperoxidase-like activity of ssDNA-assisted AuNP nanozyme to catalyze 3,3’,5,5’-tetramethylbenzidine (TMB) and achi- eved the highly sensitive and selective detection of aflato- xin B1.

The DNA triplex is specifically formed by a third homo- purine or homopyrimidine DNA strand bonded to a DNA duplex (Wang, 2015; Ni, 2018; Qi, 2020).This molecule can be combined with aptamers to fabricate an aptamer-triplex molecular switch (APT-TMS) (Gao., 2017; Wang, 2019; Zhao, 2019). In our previ- ous study, an electrochemical aptasensor based on a DNA nanotetrahedron and APT-TMS was developed to realize the sensitive and rapid detection of STX (Qi, 2020). In this work, APT-TMS showed excellent ability to im- prove the sensitivity of the aptasensor.

In this study, an aptamer, TMS, and AuNP nanozyme were combined to develop a label-free colorimetric apta- sensor for the rapid and highly sensitive detection of STX. An anti-STX aptamer designed with two pyrimidine arms and a purine chain were fabricated to form APT-TMS. STX triggered the opening of the switch, and enhanced the ca- talytic properties of the peroxidase-like AuNP nanozyme corresponded to the release of the ssDNA purine chains.

2 Materials and Methods

2.1 Materials

Glacial acetic acid and chloroauric acid (HAuCl4) were purchased from Sinopharm Chemical Reagent Co., Ltd. Hydrogen peroxide (H2O2, 30%) was purchased from Tian- jin Dingshengxin Chemical Industry Co., Ltd. Sodium ci- trate tribasic dihydrate was purchased from Beijing Jin- ming Biotechnology Co., Ltd. TMBwas purchased from Solarbio Science & Technology Co., Ltd. STX, neoSTX,gonyautoxin 1/4 (GTX 1/4), and okadaic acid (OA) werepurchased from the National Research Council of Canada.

All oligonucleotide sequenceswere synthesized by San- gon Biotech Co., Ltd. The anti-STX aptamer was first se- lected by Handy(2013) and then optimized to achi- eve higher affinity by Zheng(2015).The oligonu- cleotides are as follows:

Purine chain:5’-GAAGAAGAAGAA-3’;

Aptamer with two pyrimidine arms:5’-CTTCTTCTTC TTTTCTTCTTCTTC-3’;

Aptamer with one arm:5’-CTTCTTCTTCTTTNNNNNNNNNNNN-3’.

2.2 Instruments

UV-Vis absorption spectra were recorded with a UV- 2550 spectrophotometer (Shimadzu, Japan) and MultiskanSky microplate reader (Thermo Scientific, USA). Transmis- sion electron microscopy (TEM) images were acquired by a JEM-2100 system (JEOL, Japan). AuNPs were concen- trated using a 5425 centrifuge (Eppendorf, Germany).

2.3 Synthesis of AuNPs

Synthesis of the citrate-capped AuNPs was performedthe trisodium citrate reduction method. Briefly, 1mL of 1% HAuCl4solution and 95mL of ultrapure water were heated to boiling under vigorous magnetic stirring in a round-bottomed flask. Then, 4mL of 1% sodium citrate so- lution was added to the flask under constant heating and stirring. The color of the solution gradually changed from colorless to wine red. The AuNP solution was heated for another 10min, slowly cooled, and kept at 4℃ away from light. The resultant AuNPs were characterized by UV-Vis spectroscopy and TEM and then concentrated by centrifu- gation (12000rmin?1, 10min).

2.4 Formation of the Aptamer-Triplex Molecular Switch

APT-TMS was constructed according to our previously reported method (Qi, 2020). First, the anti-STX aptamer was attached with two pyrimidine arms (hereafter denoted aptamer-pyrimidine arms). Next, 6μL of 100μmolL?1aptamer-pyrimidine arms, 6μL of 100μmolL?1purine chain, and 10.5μL of ultrapure waterwere mixed in the buffer of 20mmolL?1NaCl+2.5mmolL?1MgCl2(pH 6.5). Finally, the mixture was heated at 95℃ for 5min and then slowly cooled to 25℃ to allow the purine chains and aptamer-pyrimidine arms to form APT-TMS in the solution.

2.5 Feasibility Verification of the Colorimetric Aptasensor

The formation and opening of APT-TMS were verified. First, 100μmolL?1aptamer with two pyrimidine arms or ap- tamer with one arm, 100μmolL?1purine chain, and ultra- pure water were mixed in the buffer. The mixture was heat- ed at 95℃ for 5min and then slowly cooled to 25℃ to formtriple-strand (APT-TMS) and double-strand hybrid systems. Finally, 30μL of the triple-strand and double-strand hybridsystem mixtures were incubated with 70μL of a TMB/ H2O2mixture. Pure AuNPs were used as a control. The ab-sorption spectra of the mixtures at 500–750nm were re- corded.

The developed colorimetric aptasensor was characterized to confirm whether it was well developed and applicable to STX detection. Briefly, 1.5μL of samples containing dif- ferent concentrations of STX (., 0, 37.5, 70, and 150nmolL?1) were mixed with 8.5μL of APT-TMS. After 60min of incubation, 50μL of 10nmolL?1AuNPs and 45μL of ultrapure water were added to the assay solution. A so- lution without APT-TMS was mixed with the AuNPs as a control. After 15min of incubation, 30μL of the reaction solution was added to 70μL of the TMB/H2O2mixture. The absorption spectra of the mixtures at 500–750nm were then recorded.

2.6 Optimization of the Key Parameters of the Developed Aptasensor

The concentrations of TMB and H2O2and the pH ofacetic acid were optimized. First, 20nmolL?1STX was in- cubated with the prepared APT-TMS for 60min to allow STX to bind with the anti-STX aptamer on APT-TMS. Then, 5μL of the STX and APT-TMS mixture were mixed with 50μL of 10nmolL?1AuNPs and 45μL of ultrapure water. After incubation for 15min, 70μL of the mixture was add- ed to 30μL of the above reaction solution containing a certain gradient concentration of TMB (., 0, 0.125, 0.25, 0.5, 1, and 2mmolL?1), a certain gradient concentration of H2O2(., 0, 5%, 10%, 15%, and 20%), and acetic acid of a certain pH (., 2.0, 3.0, 4.0, 5.0, 6.0, and 7.0). The ab- sorption spectra of the mixtures at 500–750nm and their corresponding absorption values at 650nm were recorded.

2.7 Detection of Saxitoxin Using the Constructed Aptasensor

APT-TMS was incubated with the buffer/sample con-taining a certain concentration of STX and then added with AuNPs to allow the released ssDNAs to adsorb onto the surface of the particles. The addition of TMB/H2O2to the mixture generated different colorimetric signals under the catalysis of AuNP nanozyme. The absorption spectra ofthe mixtures at 500–750nm were recorded. The correspond- ing absorbance at 650nm was used to calculate the signal change rate with the formula (A?A0)/A0×100%. A means the absorbance with various concentration of STX; A0means the absorbance without STX.

2.8 Pretreatment of Shellfish and Seawater Samples

Pretreatment of scallop samples was conducted accord- ing to the method reported byRamalingam(2019). Scallop samples were purchased from a local supermarket. The meat was removed from the shells, and all edible parts were homogenized. Then, 250mg of the homogenate was added to 1mL of 50% methyl alcohol and vortexed for 5min. The vortexed sample was centrifuged for 5min at 4000rmin?1, and the supernatant was collected and heated at 75℃ for 5min. The vortexed sample was centrifuged once more for 5min at 4000rmin?1, and the supernatant was obtained and stored at ?20℃ for further experiments.Seawater samples were centrifuged for 20min at 5000rmin?1and then passed through a 0.22μm filter. The filtrates were collected and stored at 4℃ in the dark before further analysis.

2.9 Statistical Analysis

The signal change rate (%) was calculated by the formu- la (A?A0)/A0×100%. Significant analysis was performed by thetestsand analyzed by thevalue.

3 Results and Discussion

3.1 Principle of the Colorimetric Aptasensor for Saxitoxin Detection

Scheme 1presents the principle of the colorimetric ap- tasensor. Incubation of the aptamer containing pyrimidine arms at its two terminals with purine chains forms APT- TMSWatson-Crick and Hoogsteen hydrogen bonds. In the presence of STX, the aptamer at the top of APT-TMS is specifically recognized by STX, which induces a con- formational change and triggers the opening of the switch. The purine chains are then released and adsorbed onto the surface of the AuNPs. Adsorption of ssDNAs confers ne- gative charges to the surface of the AuNPs and stabilizes the ·OH radicals of H2O2, thereby enhancing the electro- static interaction between AuNPs and TMB. Thus, the per- oxidase-like activity of the AuNP nanozyme is enhanced (Wang, 2014; Hizir., 2016; Yang, 2020). This approach allows the quantitation of STX. The con- centration of STX is positively correlated with the enhance- ment of the peroxidase-like activity of the nanozyme and increased absorbance of the solution. The analysis process could be completed within 1h.

Scheme 1 Schematic illustration of the label-free colorimetric aptasensor containing a DNA triplex molecular switch and gold nanoparticle (AuNP) nanozyme for the highly sensitive detection of saxitoxin (STX).

3.2 Characterization of the AuNPs

The prepared citrate-capped AuNPs were characterized by UV-Vis spectroscopy and TEM. As shown in Fig.1(a), the synthesized AuNPs exhibited strong absorbance at 520nm, as evidenced by the sharp absorption peak obtained at this wavelength. TEM revealed that the AuNPs are sphe- rical in shape and dispersed evenly with an average size of approximately 13nm. These results indicate the successful synthesis of the AuNPs. The concentration of the prepared AuNPs was measured using the UV-Vis spectroscopy atroom temperature and calculated to be 3.28nmolL?1the Lambert-Beer law (=2.4×108Lmol?1cm?1). The Au-NPs were concentrated to a final concentration of 10nmolL?1for further experiments.

3.3 Verification of the Feasibility of the Colorimetric Aptasensor

The formation of the APT-TMS structure was verified. As shown in Fig.1(b), when the double-strand hybrid sys- tem (duplex) coexists with the AuNPs, free DNA terminals could be adsorbed onto the surface of the AuNPs to form nanozyme with enhanced catalytic performance. Obvious differences in the catalytic performance of the pure AuNPs and duplex-AuNPs may be observed. Our design aimed to form a triple-strand hybrid system (APT-TMS). In theory, APT-TMS cannot be adsorbed onto the surface of AuNPs, so no significant difference in the catalytic performance ofthe pure AuNPs and APT-TMS-AuNPs is expected. As shown in Fig.1(c), the catalytic performance of duplex- AuNPs was much higher than that of APT-TMS-AuNPs and AuNPs, thereby indicating that APT-TMS, rather than the duplex system, had been successfully formed.

The feasibility of the colorimetric aptasensor for STX detection was determined. As shown in Fig.1(d), the per- oxidase-like activity of the AuNP nanozyme obviously in- creased, and gradient signal increments were observed as the concentration of STX increased. These findings demon- strate that the binding of STX to the aptamer disassembles APT-TMS and dramatically enhances the catalytic perfor- mance of AuNP nanozyme. The results confirm that the proposed aptasensor may be used for STX detection.

Fig.1 Characterization of AuNPs and feasibility verification of the proposed aptasensor. (a) UV-Vis spectrum and TEM imaging of the AuNPs; (b, c) Schematic illustration and characterization of the APT-TMS; (d) Feasibility verification of the colorimetric aptasensor towards STX.

3.4 Optimization of the Key Parameters of the Developed Aptasensor

Previous reports revealed that ssDNA/AuNP nanozyme show higher peroxidase-like activity than pure AuNPs and that the catalytic process between the AuNPs and TMB/ H2O2follows the Michaelis-Menten model (Liu, 2014;Hizir., 2016; Yang, 2020). Thus, the optimal concentrations of TMB and H2O2and pH of acetic acid were determined to obtain the ideal experimental conditions for STX detectionthe proposed aptasensor.

Figs.2(a, b) shows that the color intensity and absor- bance of the test solution at 650nm increased and then reached a plateau as the concentration of TMB increased to 1.0mmolL?1. Thus, 1.0mmolL?1TMB was selected for subsequent experiments. Figs.2(c, d) reveals that the color intensity and absorbance of the test solution at 650nm in- creased and then reached a plateau as the concentration of H2O2increased to 15%. Hence, 15% H2O2was selected for further experiments. Finally, Figs.2(e, f) shows that the color intensity and absorbance of the test solution at 650nm peaked when the pH of acetic acid was 4.0. This find- ing may be attributed to the inhibition of the transduction of H2O2into H2O and O2at lower or higher pH (Wang., 2010; Wang, 2017). Therefore, acetic acid with pH 4.0 was selected for the next experiments.

Fig.2 Optimization of key parameters of the developed aptasensor. (a, b) Optimization of the concentration of TMB; (c, d) Optimization of the concentration of H2O2; (e, f) Optimization of the pH values of acetic acid. Significance analysis was characterized by P value using two-tailed t-tests. The symbol ‘○’ denotes control group, ‘☆’ denotes P>0.05, ‘★’ denotes P<0.05, ‘★★’ denotes P<0.01, ‘★★★’ denotes P<0.001.

3.5 Detection of Saxitoxin

Under the optimal conditions determined above, the co-lorimetric aptasensor based on APT-TMS and AuNP nano- zyme was used for STX detection. As shown inFig.3(a), increases in STX concentration caused the absorption peakat 650nm to increase gradually. The signal change rates were calculated for quantification, and a good linear rela- tionship with the logarithm of the STX concentration in therange of 0.59–150nmolL?1was obtained (Fig.3(b)). The li- mit of detection (LOD) was calculated to be 335.6pmolL?1. As shown in Table 1, the LOD of the sensor proposed in this study is comparable with those of previously re-ported aptamer-based methods (Hou, 2016; Gao, 2017; Caglayan, 2020; Qi2020; Qiang, 2020) and much lower than those of other aptasensors, such as a temperature-assisted fluorescent aptasensor (Cheng, 2018), an LSPR aptasensor (Ha, 2019), and a SERS aptasensor (Cheng, 2019). Indeed, the LOD achieved by the proposed sensor is much lower than the current standard for STX in shellfish (80μg(100g)?1) (Fra- ga, 2012).

Fig.3 Detection of STX using the colorimetric aptasensor. (a) Absorption spectra with gradient concentrations of STX; (b) Analytical curves of the colorimetric aptasensor. CSTX, concentration of STX, unit: nmolL?1. The linear equation is y=48.00x+22.98, R2=0.9919; (c) Schematic illustration of the high selectivity of the colorimetric aptasensor; (d) Selective verification of the colorimetric aptasensor. GTX 1/4, gonyautoxin 1/4; neoSTX, neosaxitoxin; OA, okadaic acid.

Table 1 Comparison of aptasensors for STX detection reported in the literature

Note:?Converted to nmolL?1for easy comparison.

3.6 Analysis of Sensing Performance

STX and other shellfish toxins (., GTX 1/4, neoSTX, and OA) were analyzed using the constructed colorimetric aptasensor to determine the selectivity of the latter to STX. The selectivity of the aptasensor is mainly dependent on theselectivity of the anti-STX aptamer. In theory, the anti-STXaptamer can only bind with STX, which means other shell- fish toxins should not cause the disassembly of TMS (Fig.3(c)). As shown in Fig.3(d), samples containing 20nmolL?1STX alone and a mixture of 20nmolL?1STX+20nmolL?1GTX 1/4+20nmolL?1neoSTX+20nmolL?1OA led to significant signal change rates; none of the othershellfish toxins resulted in obvious signal change rates, which demonstrates that the proposed colorimetric apta- sensor possesses excellent selectivity for the discrimina- tion of STX from other shellfish toxins.

The proposed aptasensor was applied to analyze STX in shellfish and seawater samples to verify its accuracy to- ward STX in real samples. Pretreated STX-spiked shell- fish and seawater samples with STX concentrations of 5, 20, and 60nmolL?1were analyzed using the developed co- lorimetric aptasensor. As shown inTable 2, the recoveries of STX in spiked shellfish and seawater samples were in the range of 82.70%–92.66% and 88.97%–106.5%, respec- tively, and with low RSDs. These values indicate that the aptasensor has good accuracy and repeatability and may also be used to detect STX in actual samples.

Table 2 Recovery studies of the colorimetric aptasensor in shellfish and seawater samples

4 Conclusions

In summary, the APT-TMS was successfully produced, and a label-free colorimetric aptasensor based on this switch and AuNP nanozyme was developed for the highly sensi- tive and selective detection of STX. APT-TMS demon- strated sensitive responses to STX. The specific binding between STX and the aptamer led to the opening of APT- TMS, and the released purine chains were adsorbed onto the surface of AuNPs to form the AuNP nanozyme. STX was quantified by measuring the colorimetric signals ge- nerated by the AuNP nanozyme. The developed colorime- tric aptasensor demonstrated a good linear relationship be- tween the signal responses and the logarithm of the STX concentration in the linear range of 0.59–150nmolL?1, a LOD of 335.6pmolL?1, good selectivity toward STX, and good practicability for analyzing STX in shellfish and sea-water samples. The sensor showed good recoveries of82.70%–92.66% for shellfish and 88.97%–106.5% for sea- water. The simple analytical protocol offered by the deve- loped aptasensor reflects its applicability to the high- throughput screening of multiple samples. The results of this study provide a new method for the development of aptasensors to monitor other targetsthe alteration of aptamer sequences.

Acknowledgement

This research was funded by the National Natural Sci- ence Foundation of China (No. 31801620).

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April 6, 2021;

June 3, 2021;

October 26, 2021

? Ocean University of China, Science Press and Springer-Verlag GmbH Germany 2022

. E-mail: wangsai@ouc.edu.cn

(Edited by Qiu Yantao)