Bài báo khoa học về lĩnh vực Vật lý, có sự tham gia nghiên cứu của thạc sĩ Nguyễn Xuân Quy, ĐH Quốc tế Bắc Hà

//link.springer.com/article/10.1007/s11664-016-5278-7

Cong Doanh Saia, Manh Quynh Luua, Van Vu Lea, Phuong Mai Nguyena, Nguyen Hai Phama, Viet Tuyen Nguyena,*, Xuan Quy Nguyenb,Quoc Khoa Doanc,#,Thi Ha Trand
aFaculty of Physics , Vietnam National  University – University of Science, 334 Nguyen Trai, Thanh Xuan, Hanoi, Vietnam
b Bac Ha International University, Lim town, Tien Du district, Bac Ninh province.
cDuy Tan University, 182 Nguyen Van Linh, Danang, Vietnam.

dFaculty of General Sciences, Hanoi University of Mining and Geology, Tu Liem, Hanoi, Vietnam
Corresponding author:
Email: *[email protected]; #[email protected]
Tel: +84 4 35583980; Fax: +84 4 3858 4069 
Abstract:
PbS nanoparticles of 12 nm in average size were synthesized by a novel time-saving method, which is the combination of the sonochemical method and laser post-annealing process. Laser annealing proved an important step in improving crystal quality of the nanoproduct. The PbS nanoparticle’s crystalline quality and morphology were characterized with several techniques such as Raman spectroscopy, X-ray diffraction, transmission electron microscopy, diffuse reflectance spectroscopy and energy dispersive X-ray analysis. The as-produced PbS nanoproduct was then used to make a glucose sensor. Electrochemical measurements showed that the sensitivity of the glucose sensor based on PbS nanoparticles was 546.2 μAcm-2mM-1, which is much higher than the glucose sensors based on other semiconductor materials reported in literature.
Keywords: Lead sulfide; Nanoparticles; Laser annealing; Sonochemical; Glucose sensor.

• Introduction.
  As an important semiconductor of the IV-VI group, lead sulfide (PbS) has attracted considerable attention due to its unique properties such as a small direct band-gap (0.41 eV at 300 K) and large Bohr exciton radius of 20 nm [1, 2]. PbS has been widely used for a long time in many electronic applications such as sensors, phototransitors, and solar absorbers, etc [3, 4, 5]. During the past several decades, the interest of PbS to material scientists and engineers has shifted to the fabrication of nanostructures thanks to their enhanced surface and quantum properties, which cannot be attained by its bulk counterpart. Different methods were utilized to fabricate PbS nanostructures, for example: chemical deposition, electro-deposition, photochemical method and vacuum deposition, etc... [6-13]. Among these methods, solution growth techniques by the general or sonochemical method in particular showed some advantages, which included low cost, high efficiency, and ease of scaling production. However, such methods did not always offer high quality products. In this paper, we present a fast and convenient process combining sonochemical synthesis and laser treatment in order to produce high quality PbS nanoparticles efficiently. The average size of the as-prepared PbS was approximately 12 nm. The products were characterized by X-ray powder diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED) and diffuse reflectance spectroscopy. A glucose sensor was then fabricated based on the obtained PbS nanoparticles. The sensor was characterized by electrochemical measurements, which showed high sensitivity. These results suggested that PbS nanoparticles would be very promising for fabrication of low cost, high sensitivity glucose sensors.
• Experiment

                                                                                    Fig. 1. Schematic diagram of PbS/GOx/PS working electrode.

 The electrochemical cell was set up with the as-prepared working electrodes, a platinum counter electrode (CE) and a saturated Ag/AgCl reference electrode. The distance between the working electrode and counter electrode was 15 mm. Glucose concentrations were increased from 0.1 mM to 1.3 mM to simulate the fluctuation of blood sugar level in the human body. A cyclic potential voltage was applied from 0 V to 1.5 V with a step of 0.01 V and scan rate of 50 mV/s.

• Results and discussion

          Solution growth techniques from aqueous solutions by general or even the sonochemical method in particular shows some advantages, which includes low cost, high efficiency, and ease of scaling production. However, such methods do not always offer high quality products. In this work, it was found that the crystal quality was improved by laser annealing, which offers many advantages in comparison to conventional annealing methods [14]. In the following section the laser annealing treatment will be discussed in detail.
PbS is sensitive to photodecomposition, meaning that PbS nanoparticles could be turned into other substances related to oxygen such as PbO or lead sulfate, according to Yousefi et al. [15] and Blackburn et al. [16]. To eliminate the possibility of photodegradation due to laser annealing, the laser treatment was performed in a vacuum chamber in several cycles, each of which lasted for 30s. After each cycle, the sample was examined by Raman spectroscopy in situ. The power density used in the Raman measurements was also kept low to avoid heating during the acquisition time. The evolution of the Raman spectra of the PbS nanoparticles during laser annealing is shown in Fig.2. Only one sharp peak at 133 cm-1 was observed, which could be assigned to either the vibration mode of PbO [17] or a combination of the transverse acoustic (TA) and transverse optical (TO) phonon modes of PbS [18]. As the annealing process and Raman measurements were performed in vacuum, the first explanation is not likely suitable. The missing Raman characteristic peak around 960 cm-1 of lead sulfate, which usually accompanies lead oxide [16], supports the assignment of the observed Raman peak to the vibration of the PbS lattice. In addition, the exclusion of the formation of PbO or PbSO4 was confirmed by XRD and EDX measurement of the annealed sample. As seen from the Raman spectra in Fig. 2, the crystal quality of the sample clearly improved after the first cycle and continued to grow after several following cycles. Raman intensity almost reached saturation after 6 cycles, suggesting that treatment could be finished after 3 min.

                                                                                                          Fig. 2. Evolution of Raman spectra of PbS nanoparticles during laser annealing process.

                                                                                                                                            Fig. 3. XRD patterns of PbS nanoparticles before and after laser treatment.

                                                                                                                                               Fig. 4. EDX spectrum of PbS nanoparticles after laser annealing.

 

The effect of annealing was confirmed by XRD patterns of the sample before and after the laser  treatment as presented in Fig.3. The XRD pattern of the sample before post-annealing consisted of several characterized peaks of lead sulfide at 25.7°, 29.8°, 42.7°, and 50.6° corresponding to (111), (200), and (220) lattice planes of the PbS face-centered cubic structure. Besides, the XRD pattern of the sample before annealing showed a broad baseline, which is likely an indication of some amorphous material. The XRD result implies that PbS nanoparticles were formed after sonochemical process, but some amorphous PbS material also occurred in the product. The disappearance of the broad baseline in the XRD pattern after laser treatment indicates that the amorphous phase in the product was transformed into crystalline. Sharper diffraction peaks of the final product demonstrate the important role of laser annealing in improving crystal quality of PbS nanoparticles.
Also important to note is that no peaks related to PbO or PbSO4 was observed for the annealed sample, so the formation of unwanted phases such as PbO and PbSO4 was excluded. The lattice constant of the final product, determined from the XRD pattern, is a = 5.93(3) Å and is in good agreement with the value of 5.936 Å as reported in JCPDS-ICDD1993, No.5-592. A crystal size of about 11 nm was obtained from Debye – Scherrer relation [19]:

    L = 0.9 λ / Bcosθ  (1)

where B,q and l were the full width at half maximum (FWHM) in radians of the diffraction peaks, the Bragg’s diffraction angle and the wavelength for the Ka1 component of the employed copper radiation (1.54056 Å), respectively. Further, the EDX spectrum shown in Fig. 4 indicated that the PbS sample contained only Pb and S elements, other impurities if any were less than the detection limit of EDX measurement.

                                                                                                                                                                                             Fig. 5.(a) TEM and (b) HRTEM images and (c) SAED image of the PbS nanocrystals.  The inset in (b) is the fast Fourier transform pattern of the HRTEM image.

Typical TEM, HRTEM images and SAED pattern of the PbS nanoparticles are shown in Fig. 5. TEM images show that the nanoparticles had a quasi-spherical shape with an average size of about 12 nm and were well separated from each other. Figure 5b illustrated the magnified HRTEM image of the PbS nanoparticles with the lattice fringes of the (200) planes,of which the spacing of adjacent lattice planes is 2.9Å, coinciding with the value obtained from the XRD analysis. The fast Fourier transform pattern of the HRTEM image shown in the inset of Fig.5b also confirms a face-centered cubic structure of PbS nanoparticles. The SAED image in Fig.5c consists of diffraction rings, which indicates that the PbS nanocrystals are arranged in a random orientation.
The absorption of the PbS samples is obtained from the diffuse reflectance values by using the Kubelka-Munk function [20]:

 F(R) = (1-R)2/ 2R = K/S    (2)

where R, K and S  are the reflection, the absorption and the scattering coefficient, respectively. The bandgap of PbS nanoproduct is then estimated by using the following equation for a direct transition:

    αhv= A(hv – Eg)1/2    (3)

where A is a constant and Eg is the bandgap of the material. The plot of the (ahn)2 versus hnfor the PbS nanocrystals is presented in Fig.6. Extrapolating the straight portion of the graph to the energy axis at a= 0 gives the bandgap of the PbS, which is found to be 2.77 eV. This value, which is much larger than that of bulk PbS, indicates that the PbS nanocrystals exhibits the quantum confinement effect due to the reduction of crystal size to below Bohr exciton radius.

                                                                                                                                                Fig. 6.The plots of (αhν)2 versus hν of the PbS nanopowders.

Fig. 7. Role of glucose oxidase (GOx) investigated by Cyclic Voltammetry.  (a)  CV diagrams of 0.2 mM Glucose, 0.2 mM GOx and 0.2 mM glucose in 0.2 mM GOx containing solution. (b) CV diagrams of different concentrations of glucose:0.2 mM, 0.4 mM, 1 mM and 2 mM in 0.2 mM GOx.  All the CV measurements wereperformed with gold working electrode.

 

In phosphate buffered saline (PBS) 1X media, a glucose-GOx complex was formed following the specific conjugation of a glucose oxidase enzyme and a glucose molecular. Figure 7 illustrates the application of cyclic voltammetry (CV) in detecting the presence of the complexes in PBS. As shown in Fig. 7a, there was one peak at 0.14 V on the oxidation curve of the CV diagram of pure glucose in PBS, which might correspond to the glucose oxidation.  In case that the solution contained only glucose oxidase enzyme, another peak arose at 0.49 V. Other observations of GOx oxidation, investigated in 0.05M PBS with nafion modified electrode [21], showed that the peak arose at 0.4 V. The deviation in position of this peak might result from the fact that PBS 1X was used instead of nafion. When 0.2 mM of the glucose-GOx mixture was added to the PBS solution, a slight shift of peak position from 0.49 V to 0.45 V was observed. Such a change was also reported by R. Madhuet al.[22], where the shift of peak was attributed to the interaction between the glucose and GOx molecules [21, 22]. When increasing the glucose concentration, the cross-section for GOx and glucose interaction increased and resulted in the increase of the oxidation peak current of the glucose-GOx complex followed by a shift to smaller peak position (Fig.7b).

Fig. 8.CV investigation of PbS nanoparticles  based WE  at different glucose concentrations. (a)CV diagrams of 0.2 mM glucose, 2 mM glucose in 0.2 mM GOx with gold WE and of 0.2 mM glucose with the PbS based WE. (b) – CV diagrams of glucose solution at different concentrations – from 0.1 mM to 1.3 mM – with PbS-based WE. (c, d)  – Selected regions from Anodic process and Catodic process taken from the CV diagrams in (b).

 

          The CV diagram of the 0.2mM glucose observed with the PbS-modified electrode is shown in Fig. 8(a). In comparison to the CV diagram of glucose-GOx complex obtained with bare gold electrode (red line), it is recognized that the oxidation peak of glucose-GOx increased by 12 times, while a reduction peak occurred on the reduction curve at 1.14 V. Cyclic voltametry of galena was also investigated and it showed a broad peak at 0.9 V, which corresponds to the transformation of PbS to HPbO2- [23]. A shift of the reduction energy was detected in this study, which might be related to the polystyrene cover and/or to the presence of PBS. Taking note of the current amplification at 0.45 V peak of Glucose-GOx oxidation, good agreement was found with other articles [21, 22], in that the nanostructure created an electron transmitting framework, which directly transformed the reduced electron from the Glucose-GOx complex to the electrode.

Fig. 9. Glucose concentration dependence of CE current at 0.45 V – evaluated from the CV diagrams observed with and without PbS-based electrode (a) and at 1.14 V – from the CV diagram observed with PbS-based electrode (b).

 

As glucose concentration increased, the anodic current at 1.14 V (Fig. 8c) and cathodic current at 0.45 V  (Fig. 8d) increased. This can be understood as the number of the glucose molecules increased, the probability to form bonds between glucose and GOx molecules is augmented; hence, there is an improvement in the amount of the electrons transmitted to the electrode .
Cathodic currents at 0.45 V of the oxidation curves of different glucose concentrations with and without PbS-modified electrode are presented in Fig. 9a as a function of the glucose concentration. The dependence of the anodic current at 1.14 V from the reduction curve on the glucose concentration is shown in Fig. 9b. It should be noted that with the presence of PbS nanoparticles modified the electrode, the rise in the cathodic current at 0.45 V was much greater than when PbS was not used. Besides, the peak current at 1.14V showed higher sensitivity to glucose than the peak at 0.45 V. The sensor sensitivity, calculated from the intensity increase of 1.14V peak, was 546.2 μAcm-2mM-1. This is a high sensitivity in comparison with previous reported data [24-30].
The GOx molecules are usually found to be 580 – 585 residue long and have 3 sulfur atoms (S) containing hydrophilic cysteine at 164th, 206th and 512th positions. The 512th position is near the edge of the N-domain and close to the flavin adenine dinucleotide linking position. A possible mechanism responsible for the increase in the sensitivity of the biosensor would be a direct transfer of an electron from the mediator-materials to the molecules. Meanwhile, metal sulfide nanocrystals could easily link to the S atom of the organic molecules with a stable covalent bond [31]. The direct linkage of the material to the S atom from 512th residue could possibly induce the electron-transfer from GOx to the electrode and consequently explained the sensitivity enhancement in this report.

1. Conclusion

          By using a sonochemical method combined with post annealing by laser, we successfully prepared PbS nanoparticles of high quality. Laser annealing helped to save time and energy compared with conventional annealing methods and played a critical role in greatly improving the crystal quality of the as-prepared nanoproduct. The nanoproduct was then used as an electron receptor of a glucose sensor, which exhibited a much higher sensitivity when compared to the same type of sensor based on other semiconductors. The direct linkage of nanoparticles to glucose oxidases was believed to be responsible for the high sensitivity of glucose sensor in this report, however, the exact mechanism for the sensitivity enhancement of glucose sensor based on PbS nanoparticles should be subject of further studies.
Acknowledgement
The research was financially supported by Asia Research Center, Vietnam National University, Hanoi via Project CA.16.02A. The authors would like to thank the Faculty of Physics, Vietnam National University – University of Science for letting us use the equipment. One of the authors, Mr Sai Cong Doanh, would like to thank Project 911 of Vietnam International Education Department Fellowships for supporting his Phd tuition fee.
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Figure captions:
Fig. 1. Schematic diagram of PbS/GOx/PS working electrode.
Fig. 2. Evolution of Raman spectra of PbS nanoparticles during laser annealing process.
Fig. 3. XRD patterns of PbS nanoparticles before and after laser treatment.
Fig. 4. EDX spectrum of PbS nanoparticles after laser annealing.
Fig. 5.(a) TEM and (b) HRTEM images and (c) SAED image of the PbS nanocrystals.  The inset in (b) is the fast Fourier transform pattern of the HRTEM image.
Fig. 6.The plots of (αhν)2 versus hν of the PbS nanopowders.
Fig. 7. Role of glucose oxidase (GOx) investigated by Cyclic Voltammetry.  (a)  CV diagrams of 0.2 mM Glucose, 0.2 mM GOx and 0.2 mM glucose in 0.2 mM GOx containing solution. (b) CV diagrams of different concentrations of glucose:0.2 mM, 0.4 mM, 1 mM and 2 mM in 0.2 mM GOx.  All the CV measurements wereperformed with gold working electrode.
Fig. 8.CV investigation of PbS nanoparticles  based WE  at different glucose concentrations. (a)CV diagrams of 0.2 mM glucose, 2 mM glucose in 0.2 mM GOx with gold WE and of 0.2 mM glucose with the PbS based WE. (b) – CV diagrams of glucose solution at different concentrations – from 0.1 mM to 1.3 mM – with PbS-based WE. (c, d)  – Selected regions from Anodic process and Catodic process taken from the CV diagrams in (b).
Fig. 9. Glucose concentration dependence of CE current at 0.45 V – evaluated from the CV diagrams observed with and without PbS-based electrode (a) and at 1.14 V – from the CV diagram observed with PbS-based electrode (b).