Electronic Supplementary Material

Amperometric sensing of L-phenylalanine using a gold electrode modified with a metal organic framework, a molecularly imprinted polymer, and β-cyclodextrin-functionalized gold nanoparticles

Ting Wu, Xiaoping Wei*, Xionghui Ma, Jianping Li[(]

Guangxi Key Laboratory of Electrochemical and Magnetochemical Function Materials, College of Chemistry and Bioengineering, Guilin University of Technology, Guilin 541004

TEM of functionalized AuNPs

The morphology of functionalized AuNPs was characterized by transmission electron microscope (TEM). The uniform particle size and excellent dispersion of functionalized Au nanoparticles were observed in Fig. S1 with a mean diameter of around 25 nm. The results show that functionallized AuNPs was obtained through the preparation method in this paper.

Figure S1. TEM image of functionalized AuNPs

The electrochemical characterization of MI-MOF and nMI-MOF

Electrochemical impedance spectroscopy (EIS) can provide useful information concerning the impedance change at electrode surface. The impedance spectra include semicircle portion at higher frequencies and linear portions at lower frequencies, which corresponds to the electron-transfer resistance, respectively. As shown in Fig. S2, compared with bare Au electrode (curve a), the impedance of 4-ATP/Au electrode (curve b) increased slightly and subsequently increased after the electropolymerization of monomer in the presence of templates (curve c). Extraction of L-Phe caused the semicircle diameter to decrease again (curve d). It can be observed that incubation of L-Phe caused increase impedance owing to block the channel again (curve e). The EIS results were well consistent with the current changes of CV, further confirming the successful fabrication of the MI-MOF sensor.

Figure S2. AC impedances of MIP films at (a) bare gold electrode, (b) gold electrode after self-assembly, (c) MI-MOF electrode, (d) MI-MOF electrode after elution by methanol/acetic acid (5 : 3 in volume), (e) MI-MOF electrode after rebinding 6×10-11 M L-Phe in 0.02 M phosphate buffer (pH = 8).

Optimization of pH of polymerization solution

Since pH value has significant influence on polymerization of monomers, the optimum pH values of phosphate buffer were examined. After polymerization and extraction of template, the modified electrodes were employed for detection of L-Phe at the same concentration and their reduction peak current shifts (∆I) before and after exposure to templates were calculated for evaluating the sensor performance. As shown in Fig. S3, ΔI reaches the maximum value when the electrolyte pH = 8. Then, pH 8.0 was chosen.

Figure S3. Optimization of the pH values of polymerization solution, (Error bar represents the standard deviation of three repetitive experiments)

The optimization of the eluant, elution time and the rebinding time

In order to check the effect of elution on the determinations, eluents and elution time were investigated. Several eluents such as ethanol-formic acid (2:5 in volume), HAc solution (10%), NaOH 0.02mol/L, and methanol-acetic acid mixture (5:3 in volume) were investigated by measurement the current of the potassium ferricyanide on the MIP modified electrode. As a result, methanol/acetic acid mixture (5:3 in volume) has the best elution efficiency, which was selected as the eluant for the whole experiments.

The elution procedure was performed in 10 mL of methanol acetic acid mixture (3:5 in volume). The DPV peak current of the probe was recorded every 30 s in the experiment, which gradually increased and then remained constant after 350 s (Fig. S3a). Therefore, the optimal elution time was selected as 350 s. In addition, the time required for L-Phe captured by the imprinted cavities and block the electro-transfer channel was checked. The modified electrode after elution was immersed in 10 mL 0.02 M phosphate solution containing 6×10-11 mol/L L-Phe. DPV was measured every 30 s. The increasing time required to capture target molecules resulted in the decreasing peak current; this value reached a minimum of 300 s (Fig. S3b).

Figure S4 Optimization of the elution time (a) and rebind time (b) for MI-MOF

The eluant was methanol/acetic acid (5 : 3 in volume); the rebinding solution was 6×10-11 M L-Phe in 0.02 M phosphate solution (pH = 8.0). Error bar represents the standard deviation of three repetitive experiments)

Table 1 Analysis performance of various assays for detecting L-Phe

Assay / Linear range (M) / Limit of detection (M) / Reference
MIP potentiometric sensor / 2.5×10-6 - 2.5×10-2 / 1.37×10-6 / [1]
MIP electrochemiluminescence sensor / 1×10-11 - 1×10-8 / 3.1×10-12 / [2]
Bioluminescent flow sensor / 1×10-6 - 1×10--4 / 5×10-7 / [3]
MIP microspheres and flow injection chemiluminescence / 1.30×10-6 - 5.44×10-4 / 6.23×10-7 / [4]
Biosensor using UV light emitting diode / 1×10-5 - 1×10-2 / -- / [5]
MI-MOF electrochemical sensor / 2×10-12 - 6×10-10 / 3.25×10-13 / This work

-- not given

Reference:

1. Chen Y, Chen L, Bi RL, Xu L, Liu Y (2012) A potentiometric chiral sensor for L-Phenylalanine based on crosslinked polymethylacrylic acid-polycarbazole hybrid molecularly imprinted polymer. Anal Chim Acta 754:83-90. doi: 10.1016/j.aca.2012.09.048

2. Ding ZY, Li CY, Song QJ (2013) Determination of L-phenylalanine with a molecularly imprinted electrochemiluminescence sensor. Chinese J Anal Chem 41:1543-1548. doi: 10.3724/SP.J.1096.2013.21201

3. Girotti S, Ferri E, Ghini S, Budini R, Carrea G, Bovara R, Piazzi S, Merighi R, Roda A (1993) Bioluminescent flow sensor for L-phenylalanine determination in serum. Talanta 40:425-430. doi: 10.1016/0039-9140(93)80255-P

4. Qiu HM, Xi YL, Lu FG, Fan LL, Luo CN (2011) Determination of l -phenylalanine on-line based on molecularly imprinted polymeric microspheres and flow injection chemiluminescence. Spectrochim Acta A 86:456-460. doi: 10.1016/j.saa.2011.10.068

5. Arakawa T, Koshida T, Gessei T, Miyajima K, Takahashi D, Kudo H, Yano K, Mitsubayashi K (2011) Biosensor for L-phenylalanine based on the optical detection of NADH using a UV light emitting diode. Microchim Acta 173:199-205. doi: 10.1007/s00604-010-0536-5

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[(]* Corresponding author.

Tel.:+86 773 8990404. E-mail address: (X. Wei); (J. Li)