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Si surface acoustic wave device
Materials Letters 130 (2014) 14–16
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
A sensitive glucose biosensor without using glucose test strips based on ZnO/SiO2/Si surface acoustic wave device Jingting Luo a,n, Min Xie b, Pingxiang Luo b,nn, Bixia Zhao c, Ke Du c, Ping Fan a a
Institute of Thin Film Physics and Applications, Shenzhen Key Laboratory of Sensor Technology, Shenzhen 518060, China Maternity and Child Care Centers in Fujian Province, Fuzhou 350001, China c Affiliated Nanhua Hospital of University of South China, Hunan 421002, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 4 September 2013 Accepted 12 May 2014 Available online 17 May 2014
A sensitive glucose biosensor without using glucose test strips was fabricated based on a ZnO/SiO2/Si surface acoustic wave (SAW) device. The SAW devices were functionalized by immobilizing glucose oxidase onto a pH-sensitive polymer which was coated on the sensitive area of the SAW device. The sensitivity of the SAW glucose biosensor was 1.589 MHz/mM and the detection limit was lower than 6.29 10 4 mM, which was sensitive enough for glucose testing. The SAW glucose biosensor system showed good anti-interference to ascorbic acid, lactic acid and uric acid. The SAW glucose biosensor system kept a constant differential frequency for 10 h, demonstrating a good degree of reversibility and stability. & 2014 Elsevier B.V. All rights reserved.
Keywords: Biomaterials Multilayer structure Piezoelectric materials Sensors Thin films
1. Introduction Diabetes mellitus has become a worldwide public health problem that affects more than 347 million people around the world [1]. It is estimated that over 3.4 million people die from hypoglycemic and severe diabetic complications every year. According to the diabetes care and complication trials, frequent testing of glucose is crucial to prevent hypoglycemic and diabetic complications. Therefore, numerous efforts have been focused on the development of the glucose biosensor [2]. The popular glucose biosensors are based on electrochemical methods or optical methods [3,4], and most of the commercial glucose biosensors need the disposable blood sugar test strips which are expensive for tightly frequent blood glucose testing. In the past few decades, surface acoustic wave (SAW) biosensors have been widely used in biosensing field because of its advantages of high sensitivity, reliability and reusability [5,6]. Recently, SAW biosensors based on ZnO films have gained much attention [7]. ZnO has been used as various sensors and the SAW devices based on ZnO exhibits high performance [8–11]. Furthermore, the SAW biosensor based on ZnO could be integrated into portable micro-array systems and operated with simple and cheap electronic components, making it cheap and suitable for medical
n
Corresponding author. Tel.: þ 86 755 26538974; fax: þ86 755 26538735. Corresponding author. Tel.: þ 86 591 87557800; fax: þ 86 591 87555679. E-mail addresses: [email protected] (J. Luo), [email protected] (P. Luo). nn
http://dx.doi.org/10.1016/j.matlet.2014.05.073 0167-577X/& 2014 Elsevier B.V. All rights reserved.
application. However, no attempt has been made toward using SAW biosensor for glucose testing. This prompted us to develop ZnO/SiO2/Si SAW biosensor targeting glucose testing. In this work, a glucose biosensor was fabricated using ZnO/SiO2/Si SAW resonator. Through measuring the differential frequency shift of the glucose biosensor system, the glucose biosensor system can monitor the glucose fluctuation without using disposable and expensive blood sugar test strips. It was found that ZnO/SiO2/Si SAW glucose biosensor was highly sensitive, reversible and stable.
2. Experimental SiO2 layer ( 80 nm) was grown on Si wafer by dry oxidation and ZnO films ( 350 nm) were deposited onto SiO2 layer by reactive magnetron sputtering. After that, a 2-port resonator was fabricated via photolithography on ZnO/SiO2/Si structure and the schematic diagram is shown in Fig. 1(a). The parameters of interdigital transducers (IDTs) can be found in our previous work [12]. The performance of the fabricated resonator was measured by a network analyzer and the central frequency (f0) of the SAW resonator as shown in Fig. 1(b) which was 433.7 MHz. It has been reported that magnetoelastic sensor coated with a pH-sensitive polymer (P-COOH) immobilized by glucose oxidase (GOx) was successfully used for glucose testing [2]. Similarly, in this work, the SAW resonator coated with P-COOH and GOx was used to test the glucose level. The fabrication and functionalization
J. Luo et al. / Materials Letters 130 (2014) 14–16
15
Fig. 1. (a) Schematic diagram of ZnO/SiO2/Si glucose biosensor. (b) The frequency responses of SAW glucose biosensor. f0, f4, f8 and f16 denote the resonant frequencies of SAW biosensor when the is glucose applied on the sensitive area under the concentrations of 0, 4, 8 and 16 mM, respectively.
of the SAW glucose biosensor can be found in our previous work [12].
3. Results and discussion Glucose solutions with concentrations of 4, 8 and 16 mM, respectively, were alternatively applied on the sensitive area of the resonator. In the presence of glucose, the following reaction occurred on the glucose biosensor [2]: 1 GOx þ catalase Glucose þ O2 ⟹ gluconic acid 2
ð1Þ
The dissociation of gluconic acid produced H þ , resulting in a decreasing pH which was detected by P-COOH. In response to the pH decrease, the P-COOH shrink, reducing the mass loading on the SAW biosensor. The SAW resonator was sensitive to the mass loading on the path of the travelling wave [5,7]. With the decrease in mass loading, the resonant frequency of SAW biosensor increased. As shown in Fig. 1(b), when the glucose solution with the concentration of 4, 8 and 16 mM was applied on SAW biosensor, the resonant frequencies shifted to f4, f8 and f16, respectively. The resonant frequency shifted gradually, reflecting the change of mass loading, and in turn testing the change of glucose concentration in the measurement system. In order to eliminate the false result from the external interference, a reference sensor was cross-correlated with the working sensor to form a dual-channel differential measurement system, which can be found in our previous work [12]. The responses of SAW biosensor system to various glucose levels were monitored by recording the differential frequency shifts ðdΔf Þ between SAW working sensor and reference sensor. Although the human serum glucose concentration is around 3.8–6.1 mM normally, the glucose level of diabetic could decrease to 2 mM at hypoglycemic state,
Fig. 2. (a) Time-dependent differential response profiles between the working biosensor and the reference one (b) The dΔf of SAW biosensor system as a function of glucose concentration.
and could reach as high as 16 mM when hyperglycemia happened. Therefore, the SAW glucose biosensor was calibrated using glucose solutions with concentrations of 2, 4, 6, 8, 12 and 16 mM. The time-dependent dΔf to glucose solution with various concentrations is shown in Fig. 2(a). The instantaneous shift in the differential frequency occurred by the addition of glucose and rapidly reached stationary levels. The statistical total dΔf in response to glucose solutions is shown in Fig. 2(b). The frequency shift Δf due to mass loading change Δm of a SAW device can be described by [13]
Δf ¼ k
Δmf 0 2 A
ð2Þ
where k, f0, and A are the basal coefficient of the sensitive material, the intrinsic resonant frequency of the SAW biosensor and the sensitive area. The increase of glucose concentration enhanced the reaction of glucose to gluconic acid and produced more H þ , resulting in a decrease in pH and the shrinking of P-COOH. As a result, the mass loading on the SAW biosensor decreases and the resonant frequency of SAW biosensor shifts to a higher one. Plotting dΔf against the glucose concentration, a calibration curve was obtained with a regression coefficient of 0.99. The sensitivity of this glucose biosensor could be obtained by the slope of the calibration curve. As can be seen from Fig. 2(b), the SAW glucose biosensor system showed a sensitivity of 1.589 MHz/mM to glucose solution. For SAW biosensor measurement, the noise could be smaller than 100 Hz from the blank response. Defining the detection limit as the concentration that can be detected at ten times the noise level, the detection limit was lower than 6.29 10 4 mM. The mechanism of our SAW glucose biosensor was based on the mass load change of the sensitive area, which resulted from the pH changing of the testing solution. However, besides the oxidation of
16
J. Luo et al. / Materials Letters 130 (2014) 14–16
Fig. 4. dΔf response of 12 full cycles to solutions with glucose concentrations of 4 and 14 mM.
4. Conclusion
Fig. 3. dΔf of the SAW glucose biosensor system when glucose, ascorbic acid, lactic acid, and uric acid were applied on it.
ZnO/SiO2/Si SAW biosensor was used for glucose testing without using disposable and expensive glucose test strips. The SAW glucose biosensor exhibited a good linearity with glucose up to the glucose concentration of 16 mM and a high sensitivity of 1.589 MHz/mM to the glucose solution was also obtained. The SAW glucose biosensor showed good anti-interference, reversibility and stability. Acknowledgment
glucose, the local pH changing could also be contributed by ascorbic acid, lactic acid and uric acid. Therefore, the anti-interference ability of the SAW glucose biosensor to ascorbic acid, lactic acid and uric acid was examined. Fig. 3 shows the selective response of the biosensor to glucose and its interferent ascorbic acid, lactic acid and uric acid. It was found that an instantaneous shift in the frequency difference occurred by the injection of 4 mM glucose and rapidly reached a stationary level. Control experiment results showed that the dΔf for the glucose interferent was very small at the beginning and almost no differential SAW frequency shift could be observed with the time. Although the working SAW sensor could be influenced by ascorbic acid, lactic acid and uric acid, this effect was the same to the reference SAW sensor. After differential process of the SAW glucose biosensor measurement system, almost no differential SAW frequency shift could be observed by the SAW glucose biosensor measurement system through dual-channel differential testing. Thus, the anti-interference ability of the SAW glucose biosensor measurement system was good and the SAW glucose biosensor system in this work was highly specific to glucose. The reproducibility and stability of the biosensor were very important for glucose testing. Therefore, two glucose solutions with concentrations of 4 and 12 mM were alternately added on the biosensor. As shown in Fig. 4, 12 full cycles were performed without interruption within 10 h. The dΔf difference between 4 and 12 mM was constant throughout the measurement, demonstrating a high reproducibility and stability of the sensor. It has been demonstrated that physical properties of ZnO depend on the presence of grain boundaries [14,15]. The broad ZnO (002) XRD peak in Fig. S1 and the ZnO surface SEM image in the inset of Fig. S1 witnessed that ZnO films in this work were nanograined and many grain boundaries existed in the films, which is usual for ZnO films deposited by magnetron sputtering. The non-trivial behavior of biosensors in this work can be ascribed to the nanograins in ZnO films. We will perform more study on this point in our future work.
The authors are grateful for financial supports from National Natural Science Foundation of China (Grant no. 51302173), Natural Science Foundation of Guangdong Province (Grant no. S20130 40015481), Foundation for Distinguished Young Talents in Higher Education of Guangdong (Grant no. 2013LYM_0078), and Basical Research Program of Shenzhen (Grant no. JCYJ20120817163755062). Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2014.05.073. References [1] WHO, World Health Organization. 〈http://www.who.int/mediacentre/fact sheets/fs312/en/〉. [2] Cai QY, Zeng KF, Ruan CM, Desai TA, Grimes CA. Anal Chem 2004;76:4038–43. [3] Dung NQ, Patil D, Duong TT, Jung H, Kim D, Yoon SG. Sens Actuators B 2012;166:103–9. [4] Tashkhourian J, Hormozi-Nezhad MR, Khodaveisi J, Dashti R. Sens Actuators B 2011;158:185–9. [5] Gronewold TMA. Anal Chim Acta 2007;603:119–28. [6] Rodríguez-Madrid JG, Iriarte GF, Araujo D, Villar MP, Williams OA, MüllerSeber W, et al. Mater Lett 2012;66:339–42. [7] Voiculescu I, Nordin AN. Biosens Bioelectron 2012;33:1–9. [8] Meng XQ, Yang CT, Chen QQ, Gao Y, Yang JC. Mater Lett 2013;90:49–52. [9] Luo JT, Pan F, Fan P, Zeng F, Zhang DP, Zheng ZH, et al. Appl Phys Lett 2012;101:172909. [10] Water W, Chu SY, Juang YD, Wu SJ. Mater Lett 2002;57:998–1003. [11] Ali A, Ansari AA, Kaushik A, Solanki PR, Barik A, Pandey MK, et al. Mater Lett 2009;63:2473–5. [12] Luo JT, Luo PX, Xie M, Du K, Zhao BX, Pan F, et al. Biosens Bioelectron 2013;49:512–8. [13] Buttry DA, Ward MD. Chem Rev 1992;92:1355–79. [14] Straumal BB, Protasova SG, Mazilkin AA, Tietze T, Goering E, Schütz G, et al. Beilstein J Nanotechnol 2013;4:361–9. [15] Straumal BB, Protasova SG, Mazilkin AA, Schütz G, Goering E, Baretzky B, et al. JETP Lett 2013;97:367–77.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
A sensitive glucose biosensor without using glucose test strips based on ZnO/SiO2/Si surface acoustic wave device Jingting Luo a,n, Min Xie b, Pingxiang Luo b,nn, Bixia Zhao c, Ke Du c, Ping Fan a a
Institute of Thin Film Physics and Applications, Shenzhen Key Laboratory of Sensor Technology, Shenzhen 518060, China Maternity and Child Care Centers in Fujian Province, Fuzhou 350001, China c Affiliated Nanhua Hospital of University of South China, Hunan 421002, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 4 September 2013 Accepted 12 May 2014 Available online 17 May 2014
A sensitive glucose biosensor without using glucose test strips was fabricated based on a ZnO/SiO2/Si surface acoustic wave (SAW) device. The SAW devices were functionalized by immobilizing glucose oxidase onto a pH-sensitive polymer which was coated on the sensitive area of the SAW device. The sensitivity of the SAW glucose biosensor was 1.589 MHz/mM and the detection limit was lower than 6.29 10 4 mM, which was sensitive enough for glucose testing. The SAW glucose biosensor system showed good anti-interference to ascorbic acid, lactic acid and uric acid. The SAW glucose biosensor system kept a constant differential frequency for 10 h, demonstrating a good degree of reversibility and stability. & 2014 Elsevier B.V. All rights reserved.
Keywords: Biomaterials Multilayer structure Piezoelectric materials Sensors Thin films
1. Introduction Diabetes mellitus has become a worldwide public health problem that affects more than 347 million people around the world [1]. It is estimated that over 3.4 million people die from hypoglycemic and severe diabetic complications every year. According to the diabetes care and complication trials, frequent testing of glucose is crucial to prevent hypoglycemic and diabetic complications. Therefore, numerous efforts have been focused on the development of the glucose biosensor [2]. The popular glucose biosensors are based on electrochemical methods or optical methods [3,4], and most of the commercial glucose biosensors need the disposable blood sugar test strips which are expensive for tightly frequent blood glucose testing. In the past few decades, surface acoustic wave (SAW) biosensors have been widely used in biosensing field because of its advantages of high sensitivity, reliability and reusability [5,6]. Recently, SAW biosensors based on ZnO films have gained much attention [7]. ZnO has been used as various sensors and the SAW devices based on ZnO exhibits high performance [8–11]. Furthermore, the SAW biosensor based on ZnO could be integrated into portable micro-array systems and operated with simple and cheap electronic components, making it cheap and suitable for medical
n
Corresponding author. Tel.: þ 86 755 26538974; fax: þ86 755 26538735. Corresponding author. Tel.: þ 86 591 87557800; fax: þ 86 591 87555679. E-mail addresses: [email protected] (J. Luo), [email protected] (P. Luo). nn
http://dx.doi.org/10.1016/j.matlet.2014.05.073 0167-577X/& 2014 Elsevier B.V. All rights reserved.
application. However, no attempt has been made toward using SAW biosensor for glucose testing. This prompted us to develop ZnO/SiO2/Si SAW biosensor targeting glucose testing. In this work, a glucose biosensor was fabricated using ZnO/SiO2/Si SAW resonator. Through measuring the differential frequency shift of the glucose biosensor system, the glucose biosensor system can monitor the glucose fluctuation without using disposable and expensive blood sugar test strips. It was found that ZnO/SiO2/Si SAW glucose biosensor was highly sensitive, reversible and stable.
2. Experimental SiO2 layer ( 80 nm) was grown on Si wafer by dry oxidation and ZnO films ( 350 nm) were deposited onto SiO2 layer by reactive magnetron sputtering. After that, a 2-port resonator was fabricated via photolithography on ZnO/SiO2/Si structure and the schematic diagram is shown in Fig. 1(a). The parameters of interdigital transducers (IDTs) can be found in our previous work [12]. The performance of the fabricated resonator was measured by a network analyzer and the central frequency (f0) of the SAW resonator as shown in Fig. 1(b) which was 433.7 MHz. It has been reported that magnetoelastic sensor coated with a pH-sensitive polymer (P-COOH) immobilized by glucose oxidase (GOx) was successfully used for glucose testing [2]. Similarly, in this work, the SAW resonator coated with P-COOH and GOx was used to test the glucose level. The fabrication and functionalization
J. Luo et al. / Materials Letters 130 (2014) 14–16
15
Fig. 1. (a) Schematic diagram of ZnO/SiO2/Si glucose biosensor. (b) The frequency responses of SAW glucose biosensor. f0, f4, f8 and f16 denote the resonant frequencies of SAW biosensor when the is glucose applied on the sensitive area under the concentrations of 0, 4, 8 and 16 mM, respectively.
of the SAW glucose biosensor can be found in our previous work [12].
3. Results and discussion Glucose solutions with concentrations of 4, 8 and 16 mM, respectively, were alternatively applied on the sensitive area of the resonator. In the presence of glucose, the following reaction occurred on the glucose biosensor [2]: 1 GOx þ catalase Glucose þ O2 ⟹ gluconic acid 2
ð1Þ
The dissociation of gluconic acid produced H þ , resulting in a decreasing pH which was detected by P-COOH. In response to the pH decrease, the P-COOH shrink, reducing the mass loading on the SAW biosensor. The SAW resonator was sensitive to the mass loading on the path of the travelling wave [5,7]. With the decrease in mass loading, the resonant frequency of SAW biosensor increased. As shown in Fig. 1(b), when the glucose solution with the concentration of 4, 8 and 16 mM was applied on SAW biosensor, the resonant frequencies shifted to f4, f8 and f16, respectively. The resonant frequency shifted gradually, reflecting the change of mass loading, and in turn testing the change of glucose concentration in the measurement system. In order to eliminate the false result from the external interference, a reference sensor was cross-correlated with the working sensor to form a dual-channel differential measurement system, which can be found in our previous work [12]. The responses of SAW biosensor system to various glucose levels were monitored by recording the differential frequency shifts ðdΔf Þ between SAW working sensor and reference sensor. Although the human serum glucose concentration is around 3.8–6.1 mM normally, the glucose level of diabetic could decrease to 2 mM at hypoglycemic state,
Fig. 2. (a) Time-dependent differential response profiles between the working biosensor and the reference one (b) The dΔf of SAW biosensor system as a function of glucose concentration.
and could reach as high as 16 mM when hyperglycemia happened. Therefore, the SAW glucose biosensor was calibrated using glucose solutions with concentrations of 2, 4, 6, 8, 12 and 16 mM. The time-dependent dΔf to glucose solution with various concentrations is shown in Fig. 2(a). The instantaneous shift in the differential frequency occurred by the addition of glucose and rapidly reached stationary levels. The statistical total dΔf in response to glucose solutions is shown in Fig. 2(b). The frequency shift Δf due to mass loading change Δm of a SAW device can be described by [13]
Δf ¼ k
Δmf 0 2 A
ð2Þ
where k, f0, and A are the basal coefficient of the sensitive material, the intrinsic resonant frequency of the SAW biosensor and the sensitive area. The increase of glucose concentration enhanced the reaction of glucose to gluconic acid and produced more H þ , resulting in a decrease in pH and the shrinking of P-COOH. As a result, the mass loading on the SAW biosensor decreases and the resonant frequency of SAW biosensor shifts to a higher one. Plotting dΔf against the glucose concentration, a calibration curve was obtained with a regression coefficient of 0.99. The sensitivity of this glucose biosensor could be obtained by the slope of the calibration curve. As can be seen from Fig. 2(b), the SAW glucose biosensor system showed a sensitivity of 1.589 MHz/mM to glucose solution. For SAW biosensor measurement, the noise could be smaller than 100 Hz from the blank response. Defining the detection limit as the concentration that can be detected at ten times the noise level, the detection limit was lower than 6.29 10 4 mM. The mechanism of our SAW glucose biosensor was based on the mass load change of the sensitive area, which resulted from the pH changing of the testing solution. However, besides the oxidation of
16
J. Luo et al. / Materials Letters 130 (2014) 14–16
Fig. 4. dΔf response of 12 full cycles to solutions with glucose concentrations of 4 and 14 mM.
4. Conclusion
Fig. 3. dΔf of the SAW glucose biosensor system when glucose, ascorbic acid, lactic acid, and uric acid were applied on it.
ZnO/SiO2/Si SAW biosensor was used for glucose testing without using disposable and expensive glucose test strips. The SAW glucose biosensor exhibited a good linearity with glucose up to the glucose concentration of 16 mM and a high sensitivity of 1.589 MHz/mM to the glucose solution was also obtained. The SAW glucose biosensor showed good anti-interference, reversibility and stability. Acknowledgment
glucose, the local pH changing could also be contributed by ascorbic acid, lactic acid and uric acid. Therefore, the anti-interference ability of the SAW glucose biosensor to ascorbic acid, lactic acid and uric acid was examined. Fig. 3 shows the selective response of the biosensor to glucose and its interferent ascorbic acid, lactic acid and uric acid. It was found that an instantaneous shift in the frequency difference occurred by the injection of 4 mM glucose and rapidly reached a stationary level. Control experiment results showed that the dΔf for the glucose interferent was very small at the beginning and almost no differential SAW frequency shift could be observed with the time. Although the working SAW sensor could be influenced by ascorbic acid, lactic acid and uric acid, this effect was the same to the reference SAW sensor. After differential process of the SAW glucose biosensor measurement system, almost no differential SAW frequency shift could be observed by the SAW glucose biosensor measurement system through dual-channel differential testing. Thus, the anti-interference ability of the SAW glucose biosensor measurement system was good and the SAW glucose biosensor system in this work was highly specific to glucose. The reproducibility and stability of the biosensor were very important for glucose testing. Therefore, two glucose solutions with concentrations of 4 and 12 mM were alternately added on the biosensor. As shown in Fig. 4, 12 full cycles were performed without interruption within 10 h. The dΔf difference between 4 and 12 mM was constant throughout the measurement, demonstrating a high reproducibility and stability of the sensor. It has been demonstrated that physical properties of ZnO depend on the presence of grain boundaries [14,15]. The broad ZnO (002) XRD peak in Fig. S1 and the ZnO surface SEM image in the inset of Fig. S1 witnessed that ZnO films in this work were nanograined and many grain boundaries existed in the films, which is usual for ZnO films deposited by magnetron sputtering. The non-trivial behavior of biosensors in this work can be ascribed to the nanograins in ZnO films. We will perform more study on this point in our future work.
The authors are grateful for financial supports from National Natural Science Foundation of China (Grant no. 51302173), Natural Science Foundation of Guangdong Province (Grant no. S20130 40015481), Foundation for Distinguished Young Talents in Higher Education of Guangdong (Grant no. 2013LYM_0078), and Basical Research Program of Shenzhen (Grant no. JCYJ20120817163755062). Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2014.05.073. References [1] WHO, World Health Organization. 〈http://www.who.int/mediacentre/fact sheets/fs312/en/〉. [2] Cai QY, Zeng KF, Ruan CM, Desai TA, Grimes CA. Anal Chem 2004;76:4038–43. [3] Dung NQ, Patil D, Duong TT, Jung H, Kim D, Yoon SG. Sens Actuators B 2012;166:103–9. [4] Tashkhourian J, Hormozi-Nezhad MR, Khodaveisi J, Dashti R. Sens Actuators B 2011;158:185–9. [5] Gronewold TMA. Anal Chim Acta 2007;603:119–28. [6] Rodríguez-Madrid JG, Iriarte GF, Araujo D, Villar MP, Williams OA, MüllerSeber W, et al. Mater Lett 2012;66:339–42. [7] Voiculescu I, Nordin AN. Biosens Bioelectron 2012;33:1–9. [8] Meng XQ, Yang CT, Chen QQ, Gao Y, Yang JC. Mater Lett 2013;90:49–52. [9] Luo JT, Pan F, Fan P, Zeng F, Zhang DP, Zheng ZH, et al. Appl Phys Lett 2012;101:172909. [10] Water W, Chu SY, Juang YD, Wu SJ. Mater Lett 2002;57:998–1003. [11] Ali A, Ansari AA, Kaushik A, Solanki PR, Barik A, Pandey MK, et al. Mater Lett 2009;63:2473–5. [12] Luo JT, Luo PX, Xie M, Du K, Zhao BX, Pan F, et al. Biosens Bioelectron 2013;49:512–8. [13] Buttry DA, Ward MD. Chem Rev 1992;92:1355–79. [14] Straumal BB, Protasova SG, Mazilkin AA, Tietze T, Goering E, Schütz G, et al. Beilstein J Nanotechnol 2013;4:361–9. [15] Straumal BB, Protasova SG, Mazilkin AA, Schütz G, Goering E, Baretzky B, et al. JETP Lett 2013;97:367–77.