Mn-Doped ZnS Quantum Dot Imbedded Two-Fragment Imprinting

Letter pubs.acs.org/ac

Mn-Doped ZnS Quantum Dot Imbedded Two-Fragment Imprinting Silica for Enhanced Room Temperature Phosphorescence Probing of Domoic Acid Li Dan and He-Fang Wang* State Key Laboratory of Medicinal Chemical Biology and Research Center for Analytical Sciences, College of Chemistry, Nankai University, Tianjin 300071, China

Anal. Chem. 2013.85:4844-4848. Downloaded from pubs.acs.org by UNIV OF GOTHENBURG on 01/23/19. For personal use only.

S Supporting Information *

ABSTRACT: A novel strategy was presented to construct the enhanced molecularly imprinted polymer (MIP)-based room temperature phosphorescence (RTP) probe by combining the RTP of Mn-doped ZnS quantum dots (Mn-ZnS QDs) and twofragment imprinting. Two fragments or structurally similar parts of the target analytes were used as the dummy templates. Polyethyleneimine capped Mn-ZnS (PEI-Mn-ZnS) QDs, offering the binding sites to interact with the carboxyl groups of templates, were imbedded into MIPs by the hydrolysis of tetraethoxysilane. The rebinding of the target analytes to their fragments’ cavities (recognition sites) modulated the selective aggregation of Mn-ZnS QDs in QDs-MIPs and resulted in the RTP enhancement. This new method was suitable for the selective enhanced RTP detection of nonphosphorescent analytes without any derivatization and inducers. The proposed methodology was applied to construct the high selective enhanced MIPbased RTP probe for domoic acid (DA) detection. The RTP enhancement of two-fragment imprinting silica was about 2 times of one-fragment imprinting silica and 4 times of the nonimprinting silica. The two-fragment imprinting silica exhibited the linear RTP enhancement to DA in the range of 0.25−3.5 μM in buffer and 0.25−1.5 μM in shellfish sample. The precision for 11 replicate detections of 1.25 μM DA was 0.65% (RSD), and the limit of detection was 67 nM in buffer and 2.0 μg g−1 wet weight (w/w) in shellfish sample.

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the heavy-atom inducer (usually KI) and oxygen scavenger were used to obtain RTP signals.13−16 Sanz-Medel’s group reported a new strategy to anchor the heavy-atoms into MIPs for the sake of avoiding large consumption of high-purity heavy-atom salts.3,4,17−19 The heavy-atom, e.g. copper ion, was also used as the template of MIPs, where the additive that converts the targets into phosphorescent substances was demanded for RTP detection.20 Recently, we also proposed a new method based on the surface imprinting on Mn-doped ZnS quantum dots (Mn-ZnS QDs) for high selective RTP detection of nonphosphorescent analytes, without the addition of any inducers and oxygen scavengers but with the quenching RTP response.21 Herein, we presented a novel method to construct the enhanced MIP-based RTP probe by combining Mn-ZnS QDs and two-fragment imprinting (Scheme 1). Mn-ZnS QDs were used as the RTP source, which had the merits of inducer- and oxygen scavenger-free detection, and the efficient avoidance of any short-lived autofluorescence and scattering light.1 Specifically, polyethyleneimine capped Mn-ZnS (PEI-Mn-ZnS) QDs with abundant amino groups2 were imbedded into MIPs, to

oom temperature phosphorescence (RTP) has been a significant detection mode owing to its appealing advantages over fluorescence.1,2 However, traditional RTP detections always suffer from the interference from the coexisting substances. A promising way to improve the selectivity of RTP detection is to combine the RTP optosensing with the molecular imprinting technology (MIT).3,4 MIT involves the polymerization of functional and crosslinking monomers in the presence of templates.5 The subsequent removal of the templates leaves the tailored pockets of the templates, and the resultant molecularly imprinted polymers (MIPs) exhibit high-selective recognition toward that template.5 As the templates are often tightly embedded and heterogeneously distributed in the polymers’ network, there are great risks of incomplete removal and then the leakage of templates during the subsequent use of MIPs.6 The use of the alternative templates that are usually the structurally similar compounds or fragments of the targets seems to be an effective way to bypass the leakage of target templates.6−12 Besides, the alternative templates are particularly useful for imprinting those high toxic, high-cost, and/or rare targets.6,8−12 Several protocols were developed to construct the MIP-based RTP optosensing systems.3,4,13−21 The simplest combination of MIPs and RTP was using the native RTP of special targets as the detecting mode to evaluate the performance of MIPs, where © 2013 American Chemical Society

Received: September 14, 2012 Accepted: May 9, 2013 Published: May 9, 2013 4844

dx.doi.org/10.1021/ac400250j | Anal. Chem. 2013, 85, 4844−4848

Analytical Chemistry

Letter

Scheme 1. Schematic Illustration for Constructing Mn-ZnS QD Imbedded Two-Fragment Imprinting Silica for Enhanced RTP Assay of DA

WaterPro water purification system (Labconco Corporation, Kansas City, MO) was used to prepare all fresh solutions. Apparatus. The transmission electron microscopy (TEM) on a JEM-2100F field emission transmission electron microscope (JEOL, Japan) was operated at a 200 kV accelerating voltage. The samples for TEM were obtained by drying sample droplets from HAc-NaAc (10 mM, pH 3.5) dispersion onto a 300-mesh Cu grid coated with a lacey carbon film. The dynamic light scattering (DLS) measurements were carried out on laser light scattering spectrometer Zetasizer Nano ZS (Malvern, UK) at 25 °C; the detection angle is 90°. The FT-IR spectra (4000−400 cm−1) in KBr were recorded using a Magna-560 spectrometer (Nicolet, Madison, WI). The RTP measurements were performed on an F-4500 spectrofluorometer (Hitachi, Japan) equipped with a plotter unit and a quartz cell (1 cm × 1 cm). Synthesis of PIFIS. In a 50-mL flask, 0.060 g of purified PEI-Mn-ZnS QDs2 (Supporting Information) was dissolved in 10 mL of ultrapure water. Ten mL of ethanol solution of dummy templates (Table 1) was subsequently added, and the

interact with the imprinted two-fragment templates, i.e., the parts and/or structurally similar parts of the target analytes, by the hydrolysis of tetraethoxysilane (TEOS). The prepared PEIMn-ZnS imbedded fragment imprinting silica (PIFIS) displayed enhanced RTP response selectively to the targets composed of the two fragments, since the rebinding of the target analytes into the recognition sites of their fragments’ cavities modulated the selective aggregation of Mn-ZnS QDs in PIFIS. In addition, the aggregation of Mn-ZnS QDs usually resulted in the RTP enhancement2,22 due to the repairing of the defect of single Mn-ZnS QD by the neighboring ones, the location of Mn2+ distributed on the surface of Mn-ZnS QDs into inner ZnS host,23 and the more effective excitation of MnZnS QDs arising from the decreased interdot distance and increased Coulomb interaction.24 To illustrate the proposed method, domoic acid (DA), a main amnesic shellfish poisoning toxin,25−28 was chosen as the target analyte. DA caused human and animal intoxication with permanent loss of memory and even death.25−28 Due to the high toxicity and high cost of DA, the alternative templates, such as pentane-1,3,5-tricarboxylic acid (PTA)29−31 and phthalic acids,9 have been used to prepare MIPs with high affinity toward DA. In this work, we chose PTA and proline (Pro), the structurally similar parts of DA, as dummy templates to prepare the PEI-Mn-ZnS QD imbedded two-fragment imprinting silica for highly selective enhanced RTP probing of DA (Scheme 1).

Table 1. Amount of Dummy Templates in Fabrication of PIFIS

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EXPERIMENTAL SECTION Reagents. All reagents were at least of analytical grade. ZnSO4·7H2O, Mn(CH3COO)2·4H2O, and Na2S·9H2O were bought from Tianjin Kaitong Chemicals (Tianjin, China), the Second Chemicals (Shenyang, China), and Tianjin Sitong Chemicals (Tianjin, China), respectively. PEI (branched, M.W. 10000, 99%) was from Alfa Aesar (Tianjin, China), and TEOS was from Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China). PTA was from Tokyo Chemical Industry (Tokyo, Japan) and DA was from Sigma Aldrich (St. Louis, USA). L-Leucine, L-threonine, L-Pro, and L-glutamic acid were from Newprobe Biotechnology (Beijing, China). The HAcNaAc (10 mmol L−1, pH 3.5−5.5) solutions were used as buffers. Doubly deionized water (18.2 MΩ cm) obtained from a

PIFIS

PTA (mmol)

Pro (mmol)

PIFIS-1 PIFIS-2 PIFIS-3

0.15 0.30 0

0.15 0 0.30

mixture was kept vigorously stirring for 30 min. Then, 0.25 mL of TEOS was added dropwise into the mixture and kept stirring for 5 min. Finally, 0.15 mL of 25% ammonia solution was added dropwise, and the resultant mixture was kept stirring for 24 h. The nonimprinting polymer (NIP) was synthesized in parallel but without addition of the dummy templates. All the PIFIS and NIP were centrifuged and washed with absolute ethanol for 7 times, ultrapure water for 3 times, and then absolute ethanol for 2 times, and finally they were dried in vacuum. Measurement Procedure. The RTP was measured at excitation of 300 nm when the spectrophotometer was set in the phosphorescence mode. The slit width was 10 and 20 nm for excitation and emission, with the photomultiplier tube 4845



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(PMT) voltage at −950 V. PIFIS and NIP were dissolved in HAc-NaAc (10 mM) buffer to get the fresh-made stock solutions (100 mg L−1). DA stock solution (0.5 mM, in ethanol) was stocked at 4 °C. The standard DA solutions of given concentration were fresh-made in 0.5 mL of buffer. In a 4 mL quartz cell, 100 μL of a given concentration of DA standard solution was added into 2 mL of PIFIS/NIP stock solution, and then, the mixture was gently shaken for 5 min before RTP measurement. Shellfish Sample Analysis. The shellfish samples were from a local market and extracted according to a described procedure.32 Briefly, 4 g of shellfish tissue homogenate was dissolved in 16 mL of methanol−water (v/v = 1:1), and the mixture was ultrasounded for 2 h. Then, the mixture was centrifuged, and the supernatant was filtered (0.45 μm, Millipore). Due to the complex matrix of the sample, the standard addition method was applied. Briefly, a given concentration of the standard DA solution was made in 0.5 mL of supernatant, and then, 100 μL of the DA spiked sample solution was added into 2 mL of PIFIS-1 stock solution and gently shaken for 5 min before RTP detection.

even in the low pH 3.5 (black square in Figure S4b-d, Supporting Information, the relative standard deviations (RSD) for twenty measurements of RTP intensity were 3.7%, 3.2%, and 2.5% for PIFIS-1, PIFIS-2, and PIFIS-3), which was ascribed to the nature and the inner location of Mn-ZnS QDs, as well as the “proton sponge” effect of PEI. pH Effect on the RTP Response of PIFIS. As the existing states of PEI-Mn-ZnS QDs and DA were both related to the environmental pH, the interaction between PIFIS and DA was also pH dependent. The enhanced RTP intensity ΔP, normalized to the initial RTP intensity P0 (ΔP/P0), was used to evaluate the enhanced efficiency of DA toward RTP of PIFIS (Figure S5, Supporting Information). At pH 3.5−5.5, the obvious enhanced RTP responses were observed, while the most sensitive enhancement was observed at pH 3.5. However, NIP at all these pH conditions had the slightly enhanced RTP signals upon addition of DA (data not shown). Therefore, pH 3.5 was chosen for the subsequent studies. The RTP signals of PIFIS were enhanced instantly upon the addition of DA, and the enhanced signals were also stable in the subsequent 40 min (Figure S4b−d, Supporting Information, the red circle). Superiority of PIFIS-1. To evaluate the imprinting effects of PIFIS, we compared the enhanced RTP responses of PIFIS and NIP (Figures 1 and 2b). The RTP enhancement constant

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RESULTS AND DISCUSSION Synthesis and Characterization of PIFIS. The PIFIS were synthesized using PTA and/or Pro as dummy templates, PEI-Mn-ZnS QDs as binding sites, and TEOS as cross-linker (Scheme 1 and Table 1). The interaction of PEI-Mn-ZnS QDs and templates (as well as DA), the key for fine selectivity of PIFIS, was ascertained by the enhanced RTP intensity (Figure S1, Supporting Information) and the enlarged diameters (Figure S2, Supporting Information) of PEI-Mn-ZnS QDs upon the addition of templates or DA (briefly, only the data in the presence of DA were given). The amount of TEOS was crucial to the RTP intensity and fine selectivity of PIFIS. Excessive TEOS resulted in very low RTP intensity of PIFIS, because of the relatively low content of PEI-Mn-ZnS QDs in massive PIFIS and existence of some empty silica. However, too little TEOS resulted in insufficient capping of the PEI-MnZnS QDs-template complex and then low production and selectivity of PIFIS. The prepared PIFIS were the composites of PEI-Mn-ZnS QDs and amorphous silica. Taking the typical PIFIS-1 as an example, the TEM image (Figure S3a, Supporting Information) showed that the large PIFIS particles resulting from the package of silica contained several Mn-doped ZnS QDs (the small dark dots). The FT-IR spectra (Figure S3b, Supporting Information) showed the characteristic peaks of both PEI-Mn-ZnS QDs and silica. The strong and broad peak around 1078 cm−1 indicated the Si−O−Si asymmetric stretching. Other bands about 794 and 461 cm−1 also showed the Si−O vibrations. The absorptions at 2961 (C−H stretching vibration), 3417 (N−H stretching vibration), and 614 cm−1 (Zn−S vibration) revealed the existence of PEI-Mn-ZnS QDs. The FT-IR spectra of PIFIS-1 in different stages of template-removal revealed that the dummy templates were gradually and finally completely removed as the COO− absorption of the dummy templates at about 1721 and 1657 cm−1 gradually disappeared. The prepared PIFIS exhibited good RTP signals. Typically, PIFIS-1 had symmetric RTP emission at 590 nm with the maximum excitation wavelength at 300 nm (Figure S4a, Supporting Information). The RTP emission was attributed to the triple transition of Mn2+ (4T1-6A1) incorporated into the ZnS host lattice. Furthermore, the RTP signals were very stable,

Figure 1. DA concentration-dependent RTP emission spectra of PIFIS-1 and NIP (100 mg L−1) in HAc-NaAc buffer (10 mM, pH 3.5). Inset was the corresponding plots of ΔP/P0 against the concentration of DA.

(k) was calculated from the equation of ΔP/P0 = kCDA, where CDA was the concentration of DA in M, ΔP = P − P0, and P and

Figure 2. (a) RTP response of PIFIS-1 to PTA, Pro, and DA. (b) RTP response of PIFIS-1, PIFIS-2, and PIFIS-3 to DA. The concentration of PIFIS was 100 mg L−1. All the solutions were made in HAc-NaAc buffer (10 mM, pH 3.5). 4846



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P0 were the intensity of PIFIS/NIP in the presence and absence of DA. The kPIFIS‑1, kPIFIS‑2, and kPIFIS‑3 were 2.45 × 105, 9.39 × 104, and 1.04 × 105 M−1, while kNIP was 6.18 × 104 M−1; thus, the imprinting factors defined as kMIP/kNIP were 3.96, 1.52, and 1.69, respectively, for PIFIS-1, PIFIS-2, and PIFIS-3. PIFIS-1 also exhibited the enhanced RTP response to the dummy templates (Figure 2a), but the RTP response of PIFIS1 was most sensitive to DA, especially in the low concentration range. Furthermore, PIFIS-1 exhibited 2.1−2.4 times of RTP enhancement toward DA than PIFIS-2 and PIFIS-3 (Figure 2b). The two-fragment imprinting strategy allowed the selective rebinding of two parts of the targets into the cavities of PIFIS-1, respectively, and then, the targets were the binding linker of the PIFIS-1 particles (Scheme 1). For one-fragment imprinting silica, however, only one part of the target could selectively rebind into the imprinted cavities; another part of the target bound the PIFIS-2/3 mainly through the nonspecific interaction. As for QDs-NIP, the only interaction between DA and QDs-NIP was the nonspecific binding of carboxyl groups of DA and the amino groups of QDs-NIP. The selective binding of DA into the fragments’ cavities was prone to close the distance of PIFIS-1 particles, resulting in more effective aggregation of QDs in PIFIS-1, which was proved by the enlarged DLS diameters (Figure S6, Supporting Information) and the aggregated TEM image of PIFIS-1 in the presence of DA (Figure S7, Supporting Information). Consequently, the QDs embedded two-fragment imprinting silica PIFIS-1 exhibited higher imprinting effect (selectivity) and more sensitive response to DA. Figures of Merit and Selectivity of PIFIS-1. In the concentration range of 0.25−3.5 μM, the linear relationship between ΔP/P0 and CDA was observed with the regression equation ΔP/P0 = 0.2449CDA + 0.010 (where CDA was in μM, R = 0.9923, Figure 1, PIFIS-1). The precision for 11 replicate detections of 1.25 μM DA was 0.65% (RSD). The limit of detection (LOD), calculated as the concentration of DA which produced a RTP enhancement three times the standard deviation of the blank signal (normalized by P0), was 67 nM, which was lower than the LOD of PIFIS-2 (176 nM) and PIFIS-3 (118 nM). The PIFIS-1 displayed good selectivity for probing DA in the presence of main relevant metal ions, amino acids, and biomolecule. RTP enhancement of DA (0.5 μM) was not affected (±5% error was considered tolerable, Table S1, Supporting Information) by 25 mM of Na+, 12.5 mM of K+, 0.1 mM of Mg2+, 0.3 mM of Ca2+, 3 μM of Fe3+, and 2 μM of Cd2+. The tolerance of some amino acids was 15 mM of Lleucine, 1 mM of L-threonine, 6 mM of L-glutamic acid, and 3 μM of L-Pro, while bovine serum albumin was permitted at 6 μM. Application to Shellfish Sample Analysis. The PIFIS-1 was applied for the detection of DA in shellfish sample. As the supernatant matrix of shellfish sample was quite different from the pure buffer solutions, the standard additions method was carried out for the real sample analysis (Figure 3). In the range of 0.25−1.5 μM, the linear regression equation of ΔP/P0 and CDA was ΔP/P0 = 0.2203CDA +0.011 (where CDA was in μM, R = 0.9945). The slope and y-intercept were very close to those in the pure buffer, demonstrating the negligible interference of the complex biological matrix and the undetectable content of DA in shellfish sample (below the LOD of 79 nM, equivalent to DA in shellfish of 2.0 μg g−1 wet weight, which was lower than the regulatory limit for DA in shellfish of 20 μg g−1 wet weight (w/

Figure 3. RTP response of PIFIS-1 (100 mg L−1) to DA in shellfish tissue solution buffered by HAc-NaAc (10 mM, pH 3.5).

w) set by many countries). The average recovery of the spiked level of 0.25−1.5 μM was 90%.

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CONCLUSION We have developed a novel strategy to construct the enhanced MIP-based RTP probe by combining the RTP of Mn-ZnS QDs and the two-fragment imprinting protocol. The rebinding of the target analytes to the recognition sites of two fragments’ cavities modulated the selective aggregation of Mn-ZnS QDs embedded in MIPs and thus resulted in the enhanced RTP response. Although only DA was used as an example in this contribution, such methodology would be extended to Mn-ZnS QD imbedded multifragment imprinting and be adaptable for the enhanced RTP detection of other targets composed of the imprinted fragments.

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ASSOCIATED CONTENT

S Supporting Information *

Additional information as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (No. 2011CB707703), the National Natural Science Foundation of China (No. 20935001, 20977049, 21175073), and the Tianjin Natural Science Foundation (No. 13JCYBJC17000)

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Mn-Doped ZnS Quantum Dot Imbedded Two-Fragment Imprinting

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