Quantum Dot-Based Lateral Flow Test Strips for Highly Sensitive

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Article Cite This: ACS Omega 2019, 4, 6789−6795

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Quantum Dot-Based Lateral Flow Test Strips for Highly Sensitive Detection of the Tetanus Antibody Junyan Wang,† Hong-Min Meng,*,† Juan Chen,† Juanzu Liu,† Lin Zhang,*,† Lingbo Qu,† Zhaohui Li,*,† and Yuehe Lin‡ †

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Institute of Chemical Biology and Clinical Application at the First Affiliated Hospital, Henan Joint International Research Laboratory of Green Construction of Functional Molecules and Their Bioanalytical Applications, College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, P. R. China ‡ School of Mechanical and Material Engineering, Washington State University, Pullman, Washington 99164, United States S Supporting Information *

ABSTRACT: Tetanus possesses a high infection rate and remains a leading cause of infection in some developing countries. To reduce tetanus infection, point-of-care (POC) monitoring of the serum tetanus antibody level is highly desired. Here, we report fluorescent lateral flow test strips (LFTS) for simple, sensitive, and rapid detection of the tetanus antibody. By using a sandwich immunoreaction, LFTS are easily prepared. In the existence of the tetanus antibody, goat anti-human IgG (Fc)-labeled quantum dots (QDs) are captured on the test line via the formation of a sandwich structure complex of QDlabeled goat anti-human IgG (Fc)−tetanus antibody−tetanus antigens. The assay can be completed in 30 min. These LFTS possess high sensitivity for tetanus antibody detection, with a detection limit of 0.001 IU mL−1, which is 10 times lower than that of the Au nanoparticle-based test strip. The developed sensing system was also successfully applied for detection of the tetanus antibody in the human serum. Moreover, these strips can retain their specificity and sensitivity for at least 4 months when they are stored at 4 °C. All these results demonstrated that the LFTS can be used for detection of the tetanus antibody, and they exhibit great promise for in-field and POC application. immunoassay,14,15 gel electrophoresis-based immunoassay,16 and fluorescence immunoassay.17,18 Although these conventional methods show promising results for tetanus detection, there are still some shortcomings, including tedious sample pretreatment, expensive instruments, and long operation time, which are not suitable for on-site monitoring in developing countries with poor access to health facilities. Lateral flow test strips (LFTS) have been considered to be attractive sensing tools because of their rapidity, portability, low cost, and on-site detection format. This method does not need sophisticated instruments and the operation is very easy.19,20 Because the first commercial LFTS were reported for human chorionic gonadotropin detection,21 a variety of LFTS have been developed for chemical contaminants, biomolecules, drugs, hormones, biotoxins, and pathogens detection. However, though a lot of LFTS-based systems have been reported, their sensitivity limits their further application. Recently, great efforts have been made to improve the sensitivity of LFTS by choosing different labels as the signal reporter.22−24 For example, gold nanoparticles, quantum dots (QDs), colored latex beads, carbon nanoparticles, and organic fluorophores have been used to develop the LFTS system. In addition, among them, fluorescent nanoparticle-based LFTS

1. INTRODUCTION Tetanus, which is caused by the toxin produced by Clostridium tetani, has high fatal infection and fatality rate.1,2 Especially for neonatal and maternal tetanus, about 58 000 neonates and an unknown number of mothers die of tetanus every year.3 The prevention of tetanus is mainly dependent on the tetanus antibody, which can be obtained by active immunization (tetanus vaccine) or passive immunization (tetanus specific immunoglobulin). The accepted minimum level of the antibody required for protection is 0.01 IU mL−1, and the protective antibodies generated in the body of the vaccine can last for about 10 years.4 Therefore, universal implementation routine tetanus toxoid immunization of people and monitoring the level of the tetanus antibody are critical for eliminating tetanus. At present, the methods for detecting tetanus are mainly divided into two categories: in vivo neutralization test [toxin neutralization test (TNT)] and in vitro serological test. TNT is according to the action of antitoxins and toxins, comparing the serum sample with the standard antitoxin to estimate the level of the antitoxin in the serum sample.5 Because the method of TNT is time-consuming, tedious, and dangerous, this assay has been replaced by in vitro serological assay. Up to now, several in vitro serological assays have been developed for tetanus antitoxin detection, including enzyme-linked immunosorbent assay,6,7 indirect haemagglutination assay,8−11 counterimmunoelectrophoresis test,12 latex agglutination test,13 radio© 2019 American Chemical Society

Received: March 9, 2019 Accepted: April 4, 2019 Published: April 15, 2019 6789

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assay might be an ideal candidate because of its intrinsic advantages such as high sensitivity and in situ monitoring. QDs, which are fluorescent semiconductor nanocrystals, were regarded as the most promising labels for LFTS because of their unique properties, such as broad and continuous distributed excitation, high brightness, strong resistance to photobleaching, and easy surface modification. These properties make QDs an excellent reporter for the development of highly sensitive LFTS capable of simultaneous quantification of analytes. For example, the Lin group reported a fluorescent immunochromatographic test strip assay for monitoring trichloropyridinol.25 We also developed a series of QD-based LFTS for detection of nitrated ceruloplasmin26 and glutathione with a high sensitivity.27 To the best of our knowledge, no QDbased LFTS have been reported for tetanus antibody detection till date. Moreover, the toxicity of cadmium hinders the practical applicability of conventional QDs.28−30 In order to meet the requirement of environmental friendliness in practical applications, choosing cadmium-free nanoparticles as the label is urgent. Accordingly, the environment-friendly Cu:Zn−In−S/ZnS QDs may be an excellent reporter for the LFTS platform.31 They possess intrinsic advantages of QDs and unique advantages such as a larger Stokes shift, excellent chemical and thermal stability, longer fluorescent lifetime. In this work, by employing eco-friendly Cu:Zn−In−S/ZnS QDs as the signal reporter, we developed QD-based LFTS for rapid and highly sensitive detection of the tetanus antibody. There are four parts in a strip: a sample pad, a conjugate pad, a nitrocellulose (NC) membrane, and an absorbent pad. In our design, the test line and control line were modified by the tetanus antigen and standard human IgG, respectively. By recording the fluorescence intensity of QDs captured on the test line, quantitative detection of the tetanus antibody was realized. Experimental results demonstrated that our LFTS have good ability for trace detection of the tetanus antibody in the real serum sample. These QD-based LFTS may find a broad spectrum of applications in point-of-care.

Scheme 1. Schematic Illustration of LFTS Platform Detection of the Tetanus Antibody; (A) Typical Assembly of the LFTS; (B) Positive Tests Show Two Lines and Negative Tests Show Only One Line (the Control Line)

water-soluble QDs could be assigned to CO stretching of the carboxyl groups, suggesting that the 1-dodecanethiol (DDT) on the surface of QDs has been replaced by mercaptoundecanoic acid (MUA). 2.3. Feasibility of the Assay. To assess the feasibility of the QD-based LFTS, the image and fluorescence intensity on the test line under different conditions were recorded. As shown in Figure 1A, in the absence of the tetanus antibody, there is no fluorescence signal on the test line but a high fluorescence signal on the control line (test strip a). However, in the presence of the tetanus antibody, we can see a strong fluorescence signal on the test line and the fluorescence intensity enhanced with the increasing concentration of the tetanus antibody (test strip b and c). Figure 1B shows the fluorescence intensity of these test strips obtained through the reader. These results indicated that our proposed strategy can be used to detect the tetanus antibody. 2.4. Parameter Optimization. To obtain the best sensing performance of QD-based LFTS, the running buffer, blocking buffer, the amount of the QD-goat anti-human IgG (Fc) on the conjugate pad, the reaction time, and the concentration of human IgG on the control line were optimized. The running buffer was first investigated. As shown in Figure S2, by using running buffer solution containing 0.2% Tween 20, we observed a 25-fold fluorescence enhancement. Therefore, 0.2% Tween 20 buffer solution was used in the following assays. Then, the concentration of bovine serum albumin (BSA), sucrose, and Tween 20 in blocking buffer solution were optimized. As can be seen from Figure S3, the largest signal increase is observed by using 1.5% BSA, 0.5% Tween 20, and 5% sucrose in blocking buffer solution. The amount of QD-goat anti-human IgG (Fc) loaded on the glass fiber has a large influence on the fluorescence response of the test line. Therefore, the concentration of QD-goat antihuman IgG (Fc) solution was first optimized for further testing. Figure S4 shows the effect of the dilute times of the QD-goat anti-human IgG (Fc) on the fluorescence response. By using 20-fold dilution of QD-goat anti-human IgG (Fc) solution, we observed the best fluorescence enhancement (47fold) on the test line in the presence of 0.1 IU mL−1 standard

2. RESULTS AND DISCUSSION 2.1. Principle of the QD-Based LFTS. The principle of QD-based LFTS is based on the interaction between the antigen (on the test line) and the target tetanus antibody. As shown in Scheme 1, the tetanus antibody can bind to the QDgoat-anti-human IgG (Fc), forming a QD−secondary antibody−antibody complex. By employing capillary action, the complex moved upward on the NC membrane. When reaching the test line, the complex was captured by the antigen, causing a strong fluorescence intensity on the test region. Regardless of the presence of the target tetanus antibody, human IgG in the control line could combine with goat-anti-human IgG (Fc) ensuring the validity of the detection. 2.2. Synthesis and Characterization of QDs. QDs, which serve as fluorescence reporters, were synthesized by a one-pot synthetic procedure and were transferred into an aqueous solution through the ligand exchange method. The optical properties of oil-soluble QDs and water-soluble QDs were first investigated. Under the excitation of 370 nm, the oilsoluble and water-soluble QDs exhibit strong red emission at 620 and 630 nm, respectively (Figure S1A). The phase transfer process was monitored by a Fourier-transform infrared (FTIR) spectrum copy. As shown in Figure S1B, the emerging absorption band at around 1565 and 1403 cm−1 in the 6790

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Figure 1. Feasibility of the LFTS immunoassay. (A) Fluorescence imaging of QD-based LFTS for (a) 0, (b) 0.1, and (c) 0.2 IU mL−1 tetanus antibody. (B) Curves are the corresponding readout using a strip reader.

Figure 2. Fluorescence sensing of different concentrations of the tetanus antibody. (A) Fluorescence emission spectra in the presence of 0, 0.005, 0.01, 0.04, 0.08, 0.1, 0.2, and 0.3 IU mL−1 tetanus antibody (curve a−h). (B) Relationship between normalized fluorescence intensity and the target concentration. The inset shows the responses of the sensing system to the tetanus antibody at low concentration. (C) Image of the test strip with the increasing concentrations of the tetanus antibody in the range from 0 to 0.3 IU mL−1.

The effective reaction time was investigated by using 80 μL of samples containing 0.1 IU mL−1 standard tetanus antibody solution; running buffers without standard tetanus antibody solution was used as a control. As displayed in Figure S6, the signal reaches the largest at 30 min, which revealed that 30 min was enough for an effective reaction.

tetanus antibody, which was then used in the following assays. In order to get a clear line on the control line, the concentration of IgG was optimized. As shown in Figure S5, when the IgG concentration reaches to 0.1 mg mL−1, an obvious red line can be observed. Therefore, 0.1 mg mL−1 of IgG was used to modify on the control line. 6791

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2.5. Tetanus Antibody Assay. The performance of the QD-based LFTS was first examined in buffer solution. Each sample was detected three times, and the average value of these measurements was used to plot the calibration curve. Figure 2A shows the typical response to standard tetanus antibody solution with different concentrations (0.005, 0.01, 0.02, 0.04, 0.06, 0.08, 0.1, 0.2, and 0.3 IU mL−1). Standard tetanus antibody solution (0 IU mL−1) was used as the blank control. From the results, we can see that an obvious increase of the peak area was observed with the increase in tetanus antibody concentration. Meanwhile, a trace amount of the standard tetanus antibody as low as 0.005 IU mL−1 could be responded. Figure 2B describes the relationship between the value of peak areas and the various concentrations of the tetanus antibody. The inset of Figure 2B shows a good linear correlation (R2 = 0.9889) from 0.005 to 0.1 IU mL−1 with a limit of detection of 0.001 IU mL−1 based on the 3σ/slope rule, which is 10 times lower than that of gold nanoparticle-based LFTS.34 The regression equation was F/F0 = 7.4755[tetanus antibody] + 0.17521. Figure 2C shows the fluorescence images of the test line by a Sony DSLR-A300 digital camera. With increasing tetanus antibody concentrations, an obvious red fluorescence was observed on the test line. 2.6. Specificity of the Detection. For on-site assay with promising application in biological samples, a high specificity response to the target is needed. Therefore, the specificity experiments of the test strip were extended to various potentially interfering antibodies. The strip with PB buffer added was used as the blank control group. As shown in Figure 3, obvious fluorescence intensity is obtained for the 0.1 IU

2, and 4 days, 1 and 2 weeks, and 1, 2, and 4 months. Compared with strips that were stored for 0 day, the fluorescence signal of the 4 month old strips has no obvious change (Figure S7). The result indicated that the QD-based LFTS possess great stability. 2.8. Real Sample Detection. To investigate the practicality of the QD-based fluorescent strip assay, we further used the QD-based fluorescent strip to detect the tetanus antibody in the human serum samples. The human serum was obtained from the first affiliated hospital of Zhengzhou University and used under protocols approved by Life-Science Ethics Review Committee of Zhengzhou University. To reduce the interference of background fluorescence of human serum, different concentrations of the standard tetanus antibody were added to the diluted human serum samples. All the measurements were performed three times. As shown in Figure 4, the resulting calibration curve shows that the peak

Figure 4. Fluorescence response in real samples. QD-based LFTS linear response for 0.005, 0.02, 0.04, 0.06, 0.08, 0.1 IU mL−1 tetanus antibody in human serum samples.

areas versus concentrations of the tetanus antibody in the human serum samples with a dynamic concentration range from 0.005 to 0.1 IU mL−1. Also, these results agreed well with the commercial tetanus antibody kit (Table 1). In order to Table 1. Analytical Results of the Tetanus Antibody in Human Serum Figure 3. Specificity of the LFTS. The tetanus antibody, diphtheria antibody, pertussis antibody, measles antibody, and rabies antibody were all 0.3 IU mL−1, human IgG was 1 μg mL−1, and PB buffer was used as the blank control group.

method

mL−1 tetanus antibody compared to other competing antibodies, including the diphtheria antibody (0.3 IU mL−1), pertussis antibody (0.3 IU mL−1), measles antibody (0.3 IU mL−1), rabies antibody (0.3 IU mL−1), and human IgG (1 μg mL−1), suggesting that our developed system exhibits a high specificity to the tetanus antibody and can be used for the real sample. 2.7. Stability Assay of the QD-Based LFTS. Stability is a crucial parameter and should be taken into account in the design of an assay. The stability of our LFTS was verified by recording the fluorescence intensity of the same strips at 0, 1,

added (IU mL−1)

found (IU mL−1)

this method

0

0.048

kit

0.025 0.05 0 0.025 0.05

0.070 0.102 0.047 0.072 0.103

recovery (%)

RSD (n = 3; %) 1.18

95.8 104 100 106

3.26 2.53 1.82 1.42 2.48

further investigate the quantitative detection and repeatability of the results, recovery experiments are also carried out by adding standard tetanus antibody solution. From Table 1, we can see that the results obtained in real human serum show good recovery values, demonstrating good accuracy and reproducibility of this assay. At last, we also compared the performance of this QD-based test strip assay with other 6792

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precursor was added. Here, the Zn precursor was prepared by dissolving Zn(OAc)2 in the mixture solution of OAm and ODE. Following this, the temperature of the mixed solution was adjusted to 240 °C and kept for 20 min. When the reaction mixture was cooled to 60 °C, 20 mL of toluene was added into the above solution. The final product was precipitated by adding methanol and then washed three times. Water-soluble QDs was obtained by adding the hydrophilic thiol ligand to replace the initial hydrophobic surfactants.33 4.4. Preparation of QD-Goat Anti-human IgG (Fc) Conjugates. The QD−antibody conjugates were prepared by using a previously described method. Water-soluble QDs were mixed with 17.5 μL of EDC (3 mM) and 13.5 μL of NHS (7.56 mM) in PB buffer solution (pH 7.4). After 20 min, 6.9 μL of β-mercaptoethanol (7.6 mM) was added. The mixture was stirred for 5 min to disperse the solution. Subsequently, 10 μL of goat anti-human IgG (Fc) was added to the above mixture; after 2 h at room temperature, the conjugates were confined by 22.5 μL of glycine (3 mM) for 50 min. The final QD−antibody conjugates were purified by ultrafiltration for three times and stored in PB buffer solution (0.01 M PB, 0.1% Tween-20, pH 7.4) at 4 °C for further use. 4.5. Fabrication of QD-Based Fluorescent LFTS. The QD-based fluorescent LFTS contain four parts: a sample pad, a conjugation pad, a NC membrane, and an absorption pad. The sample pad (33 mm × 30 cm) and conjugation pad (7 mm × 30 cm) were made from glass fiber. The conjugation pad was prepared by dispensing a desired volume of QD−antibody onto the glass fiber pad using an XYZ Biostrip dispenser, followed by drying for 1 h at 37 °C and then storing in desiccators at room temperature. The test zone and control zone of the LFTS were prepared by the dispensing tetanus antigen (2.8 mg mL−1) and human IgG (0.1 mg mL−1) solutions on the NC membrane (2 cm × 30 cm), respectively. After drying for 1 h at 37 °C, the NC membrane was stored under dry conditions. Then, the sample pad, conjugate pad, NC membrane, and absorption pad were assembled on a support board and the master card was cut into 3.9 mm wide strips using a CT 200 Cutter. The prepared strips were stored at 4 °C for further use. 4.6. Tetanus Antibody Detection. In the detection test, different concentrations of standard tetanus antibody solution were added to the sample pad. After 30 min, the LFTS was put into an ESEQuant LFR reader, and then fluorescence intensity on the test line from QDs was recorded. These strips were also put under UV light, and the fluorescence images were captured by a Sony DSLR-A300 digital camera. 4.7. Detection of the Tetanus Antibody in the Human Serum Sample. The negative human serum samples were filtered using centrifugal filtration devices (100 K), and the solution filtrate was used to simulate the condition of serum. Spiked samples were obtained by adding a certain concentration of standard tetanus antibody solution.

methods for the detection of the tetanus antibody (Table S1). From the result, we can see that the detection limit of this strip was much lower than others, indicating its ability to distinguish negative and positive serum samples. All the results indicated that the QD-based LFTS platform holds promise for the quantitative detection of the tetanus antibody in clinical applications.

3. CONCLUSIONS Based on fluorescent immunoassay, we for the first time developed fluorescent LFTS for sensitive, rapid, and easy detection of the tetanus antibody. By choosing Cu:Zn−In−S/ ZnS QDs as the eco-friendly signal reporter, this portable system possesses a “turn-on” fluorescence response to target the tetanus antibody with a detection limit of 0.001 IU mL−1, which is much lower than the gold nanoparticle-based strips. More importantly, the assay is remarkably specific for the tetanus antibody in the presence of other competing antibodies. It has also been applied for target detection in human serum samples with satisfactory results. Overall, the simple and sensitive platform has great potential application in clinical analysis and also may help in tetanus elimination in some low-income developing countries. 4. EXPERIMENTAL SECTION 4.1. Reagents and Materials. Zinc acetate (Zn(OAc)2), copper acetate (Cu(OAc)2), indium acetate (In(OAc)3), sulfur powder, oleylamine (OAm), octadecylene (ODE), DDT, oleic acid, MUA, tetramethylammonium hydroxide, 1-ethyl-3-(3dimethylaminopropyl)-carbodiimide hydrochloride (EDC), Nhydroxy-succinimide (NHS), and Tween-20 were obtained from J&K (Beijing, China, www.jkchemical.com). The goat anti-human IgG (Fc) and human IgG were obtained from Baiaotong company (Luoyang, China, www.ablab.com.cn/ home-product.html). The tetanus antigen was obtained from Fapon Biotech Inc. (Shenzhen, China, faponbiotech.bioon.com.cn). The tetanus antibody, diphtheria antibody, pertussis antibody, measles antibody, and rabies virus antibody were all obtained from the National Institute for the Control of Pharmaceutical and Biological Products (Beijing, China, www. bzwzzx.com). The commercial tetanus antibody kit was obtained from Jining Shiye (Shanghai, China, www.shjning. com). BSA and glycine were purchased from Aladdin (Shanghai, China, www.aladdin-e.com). Phosphate buffer powder was provided by Dingguo Changsheng Biotech Co., Ltd. (Beijing, China, dingguo.bioon.com.cn). All buffers and reagent solutions were prepared with purified water, which was purified by Milli-Q system (Billerica, MA, USA, www. merckmillipore.com). 4.2. Instruments. XYZ-3030 dispenser and cutting system CT 200 were obtained from Kinbio Tech. Co., Ltd. (Shanghai, China, kinbio.bioon.com.cn). A portable fluorescence strip reader ESEQuant LFR was brought from QIAGEN (Hilden, Germany, corporate.qiagen.com). 4.3. Preparation of Cu:Zn−In−S/ZnS QDs. Cu:Zn−In− S/ZnS QDs were synthesized according to a previous report.32 Briefly, 0.014 mmol Cu(OAc)2, 0.4 mmol Zn(OAc)2, 0.4 mmol In(OAc)3, 3.2 mmol sulfur powder, 8.0 mL DDT, and 12.0 mL OAm were all added into a 100 mL flask simultaneously at room temperature. The mixed solution was heated to 220 °C under an atmosphere of Ar. After 10 min, the mixture was cooled to 100 °C and then 2 mL of the Zn

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.9b00657. Fluorescence emission and FTIR spectra of oil-soluble QDs and water-soluble QDs; effect of Tween 20, BSA 6793

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concentration in running buffers, dilute times of the QDgoat anti-human IgG and the immunoreactions time on the QD-based LFTS; stability investigation of QD-based LFTS; and a table to compare the performance between the reported methods and our proposed assay (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.-M.M.). *E-mail: [email protected] (L.Z.). *E-mail: [email protected]. Phone: (+) 86-371-67783126. Fax: (+) 86-371-67781556 (Z.L.). ORCID

Hong-Min Meng: 0000-0003-0723-1242 Zhaohui Li: 0000-0003-3946-0656 Yuehe Lin: 0000-0003-3791-7587 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21605038), the Outstanding Young Talent Research Fund of Zhengzhou University (1421316038), the Foundation for University Key Teacher by Henan Province (2017GGJS007), the Key Scientific Research Project in Universities of Henan Province (19A150048), China Postdoctoral Science Foundation (2016M602245), and the Key scientific research project of higher education of the Henan province (16A150013).

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DOI: 10.1021/acsomega.9b00657 ACS Omega 2019, 4, 6789−6795

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DOI: 10.1021/acsomega.9b00657 ACS Omega 2019, 4, 6789−6795