Electronic Supplementary Material

Fabrication of CdSe and methylene blue multilayer film for the determination of adenine and guanine in DNA

Hong Zhu, Guang-Chao Zhao*

Anhui key laboratory of Chem-Biosensing, School of Chemistry and Materials Science, Anhui Normal University, Wuhu, 241000, P R China

Electrochemical behavior of the {CdSe/MB}n multilayer electrode

The cyclic voltammograms (CVs) were recorded after each assembly of CdSe and MB. Fig. 1 shows the typical CVs of {CdSe/MB}n layers film modified electrode. Obviously, the redox currents, comes from the redox of MB, increase with numbers of CdSe/MB layer, reaching a maximum value at three layers. When the layers was over three, {CdSe/MB}n layers gave nearly the same response as the three layer, which indicated that a maximum current could be reached after 3 times of alternately adsorption. According to reference 1, CdSe’s conducting is that two processes can be distinguished: anodic abstraction of an electron from the HOMO level and cathodic injection of an electron to the LUMO level. However, with increasing CdSe/MB layer, the resistance including between layer and layer will increases, as shown in Fig. 1, which will result in the decrease of peak current. Therefore, the films of {CdSe/MB}3 was adopted in the subsequent work. From one to three layers, the oxidation current increased linearity with the increasing number of layers, which suggests the approximately same amounts of MB was immobilized on each layer.

Fig. 1 Typical CVs of the {CdSe/MB}n multilayer film in 0.1 M PBS (pH 7.0). (a) n=1, (b) n=2, (c) n=3. Scan rate was 100 mV/s. The area of the electrode is 0.196 cm2.

The effect of scan rates was investigated. With increasing scan rate, both the cathodic and anodic peak currents increase. The dependence of peak currents on scan rates showed a linear relationship with correlation coefficient of 0.9970 for ipc and 0.9954 for ipa, indicating an adsorption-controlled electrode process.

The CV response of CdSe/MB multilayer film comes from the redox of MB, whose electrochemical properties are well known in the solution phase. It has been used as a redox indicator because its formal potential is between 0.08 and −0.25 V (vs. SCE) in solution with pH 2–8 [2, 3]. In this work, the effect of the pH of the supporting electrolyte on the peak potentials of the {CdSe/MB}3 film electrode was also investigated. Fig. 2 shows the CVs the {CdSe/MB}3 electrode in different pH solution. With the increasing of solution pH from 4.0 to 9.0, the negative shift of both reduction and oxidation peak potentials was observed. The plot of formal potential versus pH shows a line with a slope of −55.2 mV pH−1 with a correlation coefficient of 0.9985. The slope is close to the reported value of −56 mV pH−1, indicating that the same proton and electron participates in the electron transfer process [4, 5].

Fig. 2 Typical CVs of the {CdSe/MB}3 film electrode in various pH PBS (from 4 to 9). Scan rate was 100 mV/s. The area of the electrode is 0.196 cm2.

Individual determination of guanine and adenine

Differential pulse voltammetry (DPV) has a much higher sensitivity. It was used to estimate the lower limit of detection and simultaneous determination of guanine and adenine. Fig. 3 shows the DPV responses of guanine or adenine alone, respectively. The anodic peak current of guanine is linearly corresponded to the concentration over the range from 0.01 mg/L to 15 mg/L. The linear regression equation is Ipa(μA) = 0.5252-1.450C (mg/L) with a correlation coefficient of 0.9927. The limit of detection (S/N = 3) is 0.3 ng/mL.

As to adenine, the linear range is from 0.03 mg/L. to 36 mg/L. The regression equation is Ipa(μA) = 0.4091-1.851C (mg/L) with a correlation coefficient of 0.9993. The limit of detection (S/N = 3) is 0.9 ng/mL.

(A)  (B)

Fig. 3 Differential pulse voltammograms of various concentrations of guanine (A) and adenine (B). The area of the electrode is 0.196 cm2.

From a to g: 0, 0.6, 1.2, 1.8, 2.4, 3.0, 3.6 mg/L in 0.1M PBS (pH7.0).

References

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