Yingchun Wanga,
Ningwei Wangd,
Xiaoni Nib,
Qianqian Jianga,
Wenming Yangc,
Weihong Huanga and
Wanzhen Xu*a
aSchool of the Environment and Safety Engineering, Jiangsu University, Zhenjiang 212013, China. E-mail: xwz09@ujs.edu.cn; Fax: +86 511 88791947; Tel: +86 511 88791919
bZhenjiang Institute for Drug Control of Jiangsu Province, Zhenjiang 212003, China
cSchool of Material Science and Engineering, Jiangsu University, Zhenjiang 212013, China
dZhenjiang Entry-Exit Inspection Quarantine Bureau, Zhenjiang 212003, P. R. China
First published on 19th August 2015
In this study, molecularly imprinted technology, combined with fluorescence measurement and computer simulation, was used to detect contaminant p-nitrophenol. Seven molecular dynamics simulations of molecular imprinting prepolymerization systems were performed to optimize the imprinting shell. Results indicated that the system with a p-nitrophenol (4-NP): 3-aminopropyltriethoxysilane (APTES): tetraethylorthosilicate (TEOS) mole ratio of 8:
8
:
12 led to the best stable template–functional monomer clusters and the hybrid SiO2 layer with CdS-like clusters on the surface of CdTe was synthesized by a simple reflux procedure. The prepared imprinted materials (CdTe@SiO2-MIPs) on the surface of silica contained CdTe nanoparticles (CdTe@SiO2) by surface imprinting and Stöber method polymerization were characterized by transmission electron microscopy, fluorescence spectroscopy, Fourier-transform infrared spectroscopy, ultraviolet–visible analysis and X-ray diffraction. A linear relationship between relative fluorescence intensity and the concentration of 4-NP was obtained with a limit of detection of 0.08 μmol L−1 and the imprinting factor (IF) was 2.23 which indicated that special binding sites with binding property to p-nitrophenol were created on the surface of the CdTe@SiO2-MIPs materials. Ultimately, the feasibility of the fluorescent materials was successfully evaluated through the analysis of 4-NP in tap water and lake water. The recoveries were above 97.3%.
Molecular imprinting is a process whereby functional monomers and cross-linking agents are copolymerized in the presence of the target analytes which act as molecular templates.8–12 The target analytes or derivatives are employed as templates to form complexes with functional monomers via covalent or non-covalent interactions, such as hydrogen bonds, ionic and/or hydrophobic interactions, around which cross-linking monomers are arranged and co-polymerized to form a rigid polymer. The target analytes are regarded as the template species to offer specific binding sites in the process of forming molecularly imprinted polymers. After removal of the template, these tailor-made polymer materials possess special shape and size of binding sites, which will exhibit a great affinity for the template molecules.13 In recent years, owing to many remarkable advantages such as high selectivity, physiochemical stability, low cost, specific recognition against the imprinted molecules14 and easy preparation, molecularly imprinted polymers (MIPs) have been widely used to many fields such as chromatographic separation, antibody mimetics, artificial receptors and in catalysis.15–19 However, many challenges still remain to be addressed. One is the low yield of high affinity sites in the process of formation of the molecularly imprinted layer.20,21 There are many studies aimed at improving the yield of high affinity binding sites including stoichiometric imprinting strategy,21,22 covalent imprinting mechanisms,23 functional monomer dimerization20 and site-selective chemical modification of molecularly imprinted polymers by covalent imprinting mechanisms.24 As a result, excess functional monomers, which offer more background sites, are often added to ensure the formation of the most stable template–functional monomer clusters so that they can increase the affinity and selectivity for the guest molecules. Empirical optimization via additional synthetic steps is a common approach to synthesize better MIPs. However, such methods are not rigorous and are time-consuming. Computational chemistry, which can be used to design the prepolymerization systems to choose the most suitable ratio of templates and functional monomers, has been introduced in MIPs to avoid functional monomer overloading and reduce preparation time.25
To further improve the selectivity and sensitivity of MIPs, fluorescence is a meaningful signalling element because of its simplicity and low detection limit. Quantum dots (QDs), have received much attention due to their unique optical properties such as high quantum yield, narrow and symmetrical spectra, broad excitation spectra and photostability.26,27 Recently, owing their special traits, QDs have been used as novel probes and combined with molecular imprinting techniques. Many researchers have produced several kinds of fluorescence probes such as ZnS:Mn2+ quantum dots,28 CdSe,29 CdZnTe alloyed quantum dots.30 Nevertheless high stability is necessary for highly sensitive detection applications. There have been some reports on coating SiO2 layers on the surface of semiconductor materials.37,38 For example, QDs which are introduced in various bio-applications are coated with a thin shell.36 However, such SiO2 shells dramatically reduces the fluorescence intensity.39 In reference work,40,41 CdTe@SiO2 was prepared using the Stöber method by adding TEOS directly into the mixture of NaHTe, Cd2+, mercaptopropionic acid, with a silica shell being formed on the surface. This method is not conducive to form good core–shell structure and can cause problems or difficulties for FL detection. In this study, a modified sol–gel method with a certain time of refluxing successfully led to QDs covered with a thin hybrid SiO2 shell.43 The hybrid SiO2-coated CdTe showed good core–shell structure and the thinness of the shell led to reduced fluorescence decrease. High FL efficiency was retained and the stability was increased as the CdS-like clusters were embedded in the SiO2 shell on the surface of QDs (hybrid SiO2-coated CdTe). Also, with the protection of the hybrid SiO2 shell, their fluorescence lifetime was longer than that of the initial CdTe materials. Thus QDs coated with a hybrid SiO2 shell should be good candidates for supersensitive applications due to their extremely high FL efficiency and good stability in solution.
The hybrid SiO2-coated QDs and computer simulation as auxiliary means in molecular imprinting technique were applied to detect 4-NP. The thioglycolic acid (TGA) functionalized CdTe QDs were synthesized in aqueous phase, and the modified sol–gel method with a simple reflux process was used to coat with a thin hybrid shell CdS-like clusters on the surface of CdTe QDs. Molecular dynamics simulations were employed to optimize the imprinting shell of the fluorescent sensor and employment of the hybrid SiO2-coated CdTe with increasing FL efficiency. Simulation of molecular structure was applied to depict the three dimensional structure to help us more fully understand the molecular interactions. The prepolymerization systems constituting of 4-NP, APTES, TEOS and ethanol were modelled through computer simulation and the binding energies between various groups were calculated. Then according to the data concerning the intermolecular forces between molecules, the optimal molar ratio of the template/monomer was selected to synthesize MIPs. Furthermore, compared with other methods, this procedure only requires the calculation of binding forces between molecules, and is therefore rapid, simple, convenient and low cost.
Fig. 1 shows the FL emission behaviors of CdTe QDs capped by TGA. As shown in Fig. 1A and B, increasing the refluxing time leads to a gradual increase in wavelength within the whole wavelength range of CdTe QDs and progressive red shifts at the wavelength onset. Accompanying these variations in wavelength, fluorescence intensity is enhanced as a function of the refluxing time up to five hours. As the fluorescence intensity and wavelength increase only slightly after three hours, the CdTe QDs obtained after 3 h of refluxing were chosen for the subsequent preparation of the CdTe@SiO2 composite particles. In Fig. 1C, increasing fluorescence followed by a decrease is observed with pH, and pH 12.0 was found to be optimal. As a consequence, 3 h refluxing time and pH = 12.0 were chosen for the following experiments. Also, considering the ultraviolet absorption peak of 4-NP in Fig. 1D and so as to prevent the occurrence of multiple frequency peaks, the wavelength of QDs at 560 nm, observed at pH = 12.0 and 3 h refluxing time, was suitable.
As multicore or mononuclear CdTe@SiO2 particles were obtained upon the use of prepared CdTe QDs, a modified sol–gel method was used to synthesize hybrid SiO2-coated CdTe particles. The seed-growth method, which is modification of the sol–gel method, led to a red shift of the peak of the QDs and the FL intensity increased as shown in Fig. 2. During reflux, the CdS-like clusters nucleated and grew into a thin layer. The factor for the increased fluorescence efficiency should be the absence of an interface between the QDs and the generated clusters. The red shift is ascribed to a reduction of quantum size effect through the formation of CdS clusters in the vicinity of the QDs.
In alkaline environment, the QDs were coated with a SiO2 layer because of partial hydrolysis of TEOS. Refluxing for 30 min led to a SiO2 shell on the QDs. A very thin silica layer was successfully integrated into the surface of the QDs in an alkaline CdTe colloidal solution with TGA and Cd2+. The crystal structure formation and corresponding core/shell heterostructures of CdTe quantum dots were characterized by X-ray investigation. Fig. 3 displays the XRD pattern of CdTe and corresponding CdTe@SiO2 nanoparticles. Fig. 3a shows that as-prepared TGA capped CdTe dots have a characteristic cubic zinc blende CdTe pattern with diffraction peaks at 23.7, 39.2 and 47.3°, which is also the dominant crystal phase of bulk CdTe, corresponding to (111), (220) and (311) planes of the reported CdTe peaks (JCPDS card no. 15-0770). Fig. 3b shows the hybrid SiO2 shell including CdS-like clusters on the surface of CdTe QDs. At 2θ value of 20.7°, there appeared a peak in CdTe@SiO2 particles, which can be indexed to SiO2 spheres. Further there are obvious shifts from CdTe to CdS structure and the three distinct diffraction peaks of CdTe QDs are weakened, due to the existence the thin silica layer CdS-like clusters.
Type | 4-NP | APTES | TEOS | Ethanol |
---|---|---|---|---|
APTES-MIP-1 | 8 | 10 | 32 | 100 |
APTES-MIP-2 | 8 | 10 | 8 | 100 |
APTES-MIP-3 | 8 | 10 | 16 | 100 |
APTES-MIP-4 | 8 | 12 | 16 | 100 |
APTES-MIP-5 | 8 | 8 | 10 | 100 |
APTES-MIP-6 | 8 | 8 | 12 | 100 |
APTES-MIP-7 | 8 | 8 | 16 | 100 |
As a comparison, APTES-MIP-1, APTES-MIP-2 and APTES-MIP-3 were selected to investigate the effect of concentration of cross-linking agents on the templates and functional monomer complexation. RDFs in Fig. S1(A and B)† of APTES-MIP-1 and APTES-MIP-2 show a relatively poorer interaction between O1 and N1 than interaction of APTES-MIP-3 in Fig. 5A, most probably due to TEOS being at too high or low concentration, respectively. Too high a concentration would hinder APTES to move freely whereas too low a concentration would lead to lower 4-NP interaction with APTES. APTES-MIP-3 shows a more favorable radial distribution function and thus the prepolymerization mixture of APTES-MIP-3 is best. Based on this result, fixing the ratio of templates and cross linking agents, we further studied APTES-MIP-3, APTES-MIP-4 and APTES-MIP-7 as another group of mixtures to optimize the concentration of functional monomers. The RDFs in Fig. 5A, B and D show their interaction between O1 and N1. It is obvious that increasing the amount of APTES does not enhance the interaction between APTES and 4-NP but when the ratio of 4-NP and APTES was decreased to 8:
8 as in APTES-MIP-7, an improvement is seen. A possibility was that the higher concentration of cross-linking agent trapped APTES by the influence of steric effects. Now setting the APTES level, in order to get the optimum proportion of these mixtures, we further compared APTES-MIP-5, APTES-MIP-6 and APTES-MIP-7. When the ratio of 4-NP and APTES was 8
:
8, the acting force of the binding sites was little affected by varying the TEOS level. What caused this phenomenon might be the self-assembly of APTES under the lower concentration cross-linker, and a mass of cross-linking agent TEOS may be self-assembly to form a rigid structure, then hinder the function between monomer PATES and template 4-NP. The RDFs of APTES-MIP-5 (6 and 7) in Fig. S1C† and 5C and D show that the interaction between O1 and N1 of APTES-MIP-6 is an optimal uniform distribution around 3.0 Å with the best result obtained when the ratio of 4-NP, APTES and TEOS was 8
:
8
:
12 as shown in Fig. 5C for APTES-MIP-6. The interactions between O1 and N1, with sharp and high peaks are around 2.85 Å, indicate formation of a strong hydrogen bond. However, in order to prove the correctness of the results, we chose four prepolymerization mixtures corresponding to APTES-MIP-3, APTES-MIP-4, APTES-MIP-6, APTES-MIP-7 as contrast experiments to synthesize CdTe@SiO2-MIPs by the same method but with different ratios.
For further validation, CdTe@SiO2-MIPs were synthesized by Stöber method. The structures and morphological characteristics of the resulting MIP materials were investigated by transmission electron microscopy (TEM) as shown in Fig. 7. The products are core–shell particles but hybrid SiO2-coated CdTe cores and imprinting layers on the particles was always clearly observed, Fig. 7A and B show relatively poorly characterised particles for APTES-MIP-3 and APTES-MIP-4, corresponding to RDFs in Fig. 5A and B. By contrast Fig. 7C and D show relatively good images of APTES-MIP-6 that is synthesized with the optimum ratio corresponding to the RDF in Fig. 5C. The shape of the APTES-MIP-6 was close to spherical with size in the range of 50.0 ± 3.0 nm after coating with silica and the obtained hybrid SiO2-coated CdTe core is about 5.0 nm in size. The hybrid SiO2-coated CdTe core and imprinting layer on each particle can be clearly observed. Fig. 7E and F, corresponding to APTES-MIP-7 show better images for APTES-MIP-3 and APTES-MIP-4, but worse than for APTES-MIP-6. The dispersity of APTES-MIP-6 in the solvent and the dispersion degree of quantum dots in the imprinted layer was clearly optimal. This result again certifies the correctness of the molecular dynamics simulations.
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Fig. 7 TEM images of APTES-MIP-3 (A), APTES-MIP-4 (B), APTES-MIP-6 (C and D) and APTES-MIP-7 (E and F). |
Both TEM images and the ultraviolet absorption experiments pointed to APTES-MIP-6 as the optimal mixture. Therefore all following experiments were performed with the ratio of APTES-MIP-6 to synthesize CdTe@SiO2-MIPs. CdTe@SiO2-NIPs were synthesized by the same method but without 4-NP templates.
Fig. 2c and d are the FL spectra of CdTe@SiO2-NIPs and CdTe@SiO2-MIPs. The FL intensity of CdTe@SiO2-MIPs and CdTe@SiO2-NIPs are slightly lower than that of hybrid SiO2-coated CdTe. In comparison with the FL spectrum of CdTe@SiO2, the PL peak of CdTe@SiO2-MIPs is little red shifted, which arise from thickness of the imprinted silica layer and the large electric field. Another may due to energy loss of the CdTe QDs when synthesizing the MIPs might have little duty for this phenomenon.
To ensure the successfully compound CdTe@SiO2-MIPs on the surface of the CdTe@SiO2, the FT-IR spectra of CdTe@SiO2-MIPs, CdTe@SiO2-NIPs and CdTe@SiO2-4-NP are detected with the KBr disks and are compared in Fig. 8. The broad and strong peaks appeared at 1103 cm−1 and 791 cm−1 are the Si–O–Si and Si-O in Fig. 8, respectively. According to a ref. 33 peak of N–H is around 1500 to 1900 cm−1 when the bending vibration took place. The peaks of 1589 and 1556 cm−1 in Fig. 8a–c are the N–H of 4-NP and N–H of APTES, respectively. But compared with CdTe@SiO2-4-NP (a), the peaks are absent or are only little in CdTe@SiO2-MIPs (b). The N–H band around 1556 cm−1 in Fig. 8(a and b), resulting from APTES, proved successful synthesis CdTe@SiO2-MIPs and CdTe@SiO2-NIPs on the surface of the CdTe@SiO2. The peaks at 1495 cm−1, 1390 cm−1, 856 cm−1 and 642 cm−1 were the bands CC, O
N
O, C–N and C–H of CdTe@SiO2-4-NP. These peaks missed in CdTe@SiO2-MIPs to prove the imprinted sites existing. And the other aspects that the spectra of CdTe@SiO2-MIPs and CdTe@SiO2-NIPs were same also can prove the successful synthesis.
F0/F = 1 + KSV[Q] |
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Fig. 10 Fluorescence emission spectra of CdTe@SiO2-MIPs (A) and CdTe@SiO2-NIPs (B) at different concentrations of 4-NP in aqueous solution and the corresponding Stern–Volmer curve (insets). |
In the quenching kinetics equation F0 and F are the FL intensities of the CdTe@SiO2-MIPs before and after adding the template, respectively. KSV is the Stern–Volmer constant, and [Q] is the concentration of quencher. In Fig. 10A, in the concentration range of 10–60 μmol L−1, the fitting equation has a linear relationship with a correlation coefficient of 0.99734 for CdTe@SiO2-MIPs, which verifies that the synthesized imprinted materials has good recognition for the template 4-NP through the quenching degree of the fluorescence intensity. The linear regression equation of CdTe@SiO2-MIPs is F0/F = 0.99055 + 0.031189C4-NP. The lower limit of quantitation is calculated by 3σ/K, and corresponds to 0.08 μmol L−1. Compared with the fluorescence sensor based on Mn-doped ZnS quantum dots,35 the present fluorescence sensor provides a wide linear range and high selectivity. The binding sites on the surface of the CdTe@SiO2-MIPs are efficient for the template to reunite with the CdTe@SiO2-MIPs. CdTe@SiO2-NIPs materials showed the same quenching phenomenon with a correlation coefficient of 0.99115. The imprinting factor (IF) was 2.23 and the peak positions of CdTe@SiO2-MIPs and CdTe@SiO2-NIPs materials were little affected on addition of various amounts of 4-NP. The non-covalent interactions between the specified atoms through electron transfer should be responsible for the quenching phenomenon.
The results indicated when the template was added into the CdTe@SiO2-MIPs materials, the FL intensity of the materials decreased more dramatically than for CdTe@SiO2-NIPs, so that the MIPs showed better detection.
Sample | Spiked/μM | Found/μM | Recovery (%) | RSD (%) |
---|---|---|---|---|
Tap water | 0.0 | 0.0 | ||
Tap water | 7.5 | 7.4 | 98.7 | 4.8 |
Tap water | 25.0 | 25.2 | 100.8 | 2.4 |
Tap water | 50.0 | 50.1 | 100.2 | 1.6 |
Lake water | 0.0 | 0.0 | ||
Lake water | 7.5 | 7.3 | 97.3 | 6.4 |
Lake water | 25.0 | 25.8 | 103.2 | 5.1 |
Lake water | 50.0 | 51.1 | 102.2 | 2.1 |
The recovery rates are between 97.3 and 103.2%, and the RSDs are relatively low. Thus the fluorescence sensor prepared in this work could be used to accurately measure 4-NP in the water environment.
In the experiments, all fluorescence detections were performed under the same conditions: the excitation wavelength was set at 365 nm. The slit widths (5 nm) were kept constant within each data set. The PMT voltage was allowed to vary: 500, 550 V for Fig. 1A and B, respectively; 700 V for the other measurements.
The freshly prepared precursor NaHTe was synthesized with 125.0 mg tellurium powder (Te), 200.0 mg sodium borohydride (NaBH4) and 10.0 mL deionized water. Te and NaBH4 were added into a 15.0 mL vitreous bottle and 10.0 mL of deionized water was added. The mixture was put into ice-bath and allowed to stir to give a milky dispersion.
Separately, we added 553.0 mg cadmium chloride (CdCl2·2.5H2O) and 284.0 mg thioglycolic acid (TGA) into 175.0 mL deionized water and adjusted the mixture to pH = 12.0 with 1 M NaOH, and strongly stirred under the protection of nitrogen for 30 min. Then 6.0 mL of the fresh solution of NaHTe was added to this mixture quickly, and reacted at 100 °C with stirring and refluxing for several hours. A stable aqueous solution of thioglycolic acid capped CdTe QDs were obtained, which was used in the next step after 7 days.
First using molecular dynamics, the molecular structures and the energy of the template 4-NP, APTES and TEOS were optimized before molecular simulation of the prepolymerization, using Material Studio 7.0 Windows (Accelrys Inc., San Diego, CA92121, USA) software in the model with all the optimized systems and the simulation procedures using the COMPASS force field. Then the systems of prepolymerization were built and optimized with the construction order of the amorphous cell. In all optimizations the temperature was kept at 298 K and the density was stipulated at 1.3 g mL−1. The seven group prepolymerization mixtures were optimized with the method as for the molecules. The primary procedures we used were the SMART MINIMIZER in the Discover Tools, the iterations were set at 20000. These seven kinds of prepolymerization systems were built with different mole ratios of the materials as shown in Table 1.
Through the optimized procedure, these mixtures might become unstable. In order to avoid this we finished the molecular dynamics simulation procedures of these prepolymerization systems using SMART MINIMIZER. The simulation procedure not only could ensure the prepolymerization systems to be optimized, but also could make these systems balance relatively in the energy state. In the first place, we used the Dynamics order of the Discover module to adopt the NVT molecular dynamics system, the COMPASS force field in the opened Dynamics taskbar, to make the prepolymerization system balance after 200 ps at 298 K. The VDW and coulombic non-bonding interactions used the atom-based and Ewald methods to calculate sums, respectively. These summation methods were made over 200000 steps. The Radial Distribution Function (RDF) was used to analyze the monomer distribution around the template with the change of the cross-linking.
As a comparison, the non-imprinted polymer materials (CdTe@SiO2-NIPs) were synthesized with the same method but in the absence of template molecules.
10.0 mg of pre-prepared CdTe@SiO2-MIPs, CdTe@SiO2@-NIPs and also CdTe@SiO2-MIPs without washing the template 4-NP were dissolved in 100.0 mL deionized water and then sonicated until the solid disappeared. Then we scanned these substances at room temperature. For 4-NP detection we measured fluorescence at different concentrations (10.0, 20.0, 30.0, 40.0, 50.0, 60.0 μmol L−1) in the presence of 5.0 mg CdTe@SiO2-MIPs in 10.0 mL 4-NP solution with stirring until the nanoparticles dissolved. The fluorescence intensity was then detected.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra06889d |
This journal is © The Royal Society of Chemistry 2015 |