Using Thermolytic Solution of Anionic-decorated Graphene Quantum Dots (GQDs) as Fluorescence Turn on-off Sensor for Selective Screening Test of Metal Ions

Introduction

It is well known that graphene has found widespread applications in many diverse fields in physics, chemistry, material science, and biology. It has been shown that when a graphene sheet is small enough, its properties can vary significantly. At a size less than 100 nm, a new kind of materials called graphene quantum dots (GQDs) is produced¹. GQDs usually contain one or two layers of graphene with a size distribution mainly in the range of 3-20 nm². GQDs are highly luminescent as a result of the quantum confinement and edge effects³. The amounts of oxygen-containing groups, structural defects, and doping elements also contribute considerably to their luminescence properties⁴. They have higher surface area, larger diameter, and better surface grafting properties compared with the conventional quantum dots (QDs) or carbon dots (CDs)⁵. In addition, GQDs show excellent water solubility, low toxicity, high stability, and good biocompatibility. They are considered to be a promising material to replace the commonly used semiconductor nanocrystals for a number of biosensing and bioimaging related applications^6-8.

Glycine (GLY) functionalized graphene quantum dots (GLY-GQDs) by a simple and green pyrolysis method employing ethylene glycol as carbonsource, GLY as functional molecule was reported⁹. The as-synthesized GLY-GQDs exhibited excellent water solubility with about 22% fluorescence quantum yield. The fluorescence of GLY-GQDs was intensively quenched by Ce⁴⁺via forming non-luminescent complexes of GLY-GQDs-Ce⁴⁺in the presence of ascorbic acid. GQDs were also prepared by a simple carbonization method¹⁰. It was found that Ce⁴⁺could oxidize GQDs to produce a relatively intense chemiluminescence (CL) emission. It was attributed to the radiative recombination of oxidant-injected holes and thermally excited electrons in the GQDs. In order to show the analytical application potential of GQDs-Ce⁴⁺CL system, it was applied to the determination of uric acid. Simple fluorescent probe to detect ascorbic acid based on “off-to-on” mechanism of GQDs in the presence of Cu²⁺ion was developed¹¹. The fluorescence of GQDs was greatly quenched by Cu²⁺. Upon addition of ascorbic acid, Cu²⁺was reduced to Cu⁺and the interaction of Cu²⁺-GQDs was destroyed, which inhibited the quenching and thus leading to fluorescence recovery of GQDs. An effective approach to produce GQDs was developed¹², which based on the cutting of graphene oxide (GO) powder into smaller pieces and being reduced by a green approach, using sodium polystyrene sulfonate as a dispersant and L-ascorbic acid as the reducing agent, which is environmentally friendly. Then, the as-prepared GQDs were further used for the detection of heavy metal ions, Pb(II).

A facile method for the highly sensitive and selective sensing of biothiols based on GQDs was also developed¹³. They reported that GQDs emitted strong blue fluorescence in an aqueous buffer solution. It was observed that Hg(II) could efficiently bind with GQDs and quench its fluorescence intensity. When the biothiol compounds (glutathione, cysteine, or homocysteine) were added to the assay mixture of GQDs and Hg(II), it bound with Hg(II). Hg²⁺–GQD complex dissociated, and the fluorescence turn-on signal was detected.Hormozi-The as-prepared carbon dots (CDs) contain phenol groups on their surfaces was reported¹⁴, and due to the special coordination interaction of phenol groups with Fe³⁺ ions, this sensing system exhibits excellent sensitivity and selectivity toward Fe³⁺. In order to detect Fe²⁺ ions as well, a specific amount of H₂O₂ was introduced to the detection system to transform Fe²⁺ to Fe³⁺ of which the later one is the determinable form of iron. The boron-doped graphene quantum dots (B-GQDs) was synthesized¹⁵. The fluorescence intensity of the B-GQDs has been found to be very sensitive to the presence of Fe³⁺in the system but not to other possible coexisting metal ions, showing the high selectivity of this Fe³⁺ assay and highlighting that there is no need for tedious procedures to separate Fe³⁺ from other metal ions prior to the assay. Sulfur-doped graphene quantum dots (S-GQDs) with stable blue-green fluorescence by one-step electrolysis of graphite in sodium p-toluenesulfonate solution was synthesized¹⁶. Doping with S atoms improved the electronic properties and surface chemical reactivities of GQDs, which was beneficial for chemically binding with Fe³⁺forming S-GQDs-Fe³⁺. The S-GQDs emitting strong blue-green fluorescence were used as a novel fluorescence probe for the highly sensitive and selective detection of Fe³⁺. New class of dopamine-functionalized GQDs (DA-GQD) using one-step EDC/NHS coupling reaction for Fe³⁺detection was synthesized¹⁷. The functionalization between -COOH group of GQD molecule and amine group of dopamine results in the strong complexation interaction between catechol moiety of DA-GQD and Fe³⁺. The further oxidation of dopamine by Fe³⁺enhances the sensitivity and selectivity of sensing probes.The synthesis of D-penicillamine functionalized graphene quantum dots (DPA-GQD) through the thermal pyrolysis of citric acid and D-penicillamine was reported¹⁸. The resulting functionalized GODs exhibit ultra-high fluorescence intensity and sensitive and ultra-selective response to Fe³⁺. The nanosensor based on the GODs was developed for fluorescent detection of Fe³⁺in the iron supplement oral liquids.

This research study was aimed toward insights for preparation of GQDs via simple thermolysis of citric acid, and to well develop multi-functionalized GQDs by simple decoration with some anionic salts during thermolysis, expecting it would serve indirectly as potential doping carbon based material. Effects of experimental parameters including solution pH, weight of citric acid, heating temperature, and various kinds of anionic salts associated with GQDs affecting their fluorescence intensity were also optimized. Then, the as-prepared  and decorated GQDs were preliminary subject to perform selective analysis of various metal ions in aqueous solution.

Materials and Methods

Chemicals

All chemicals used were AR grade from QRec^TM (New Zealand) including citric acid, sodium hydroxide, sulfuric acid, perchloric acid, boric acid, hydrochloric acid, nitric acid, phosphoric acid, acetic acid, sodium acetate. Potassium dihydrogenphosphate and dipotassium hydrogen phosphate were from LOBA (India). Paraffin oil was purchased from Sigma-Aldrich (USA).

Instruments

Fluorescence Spectrophotometer, Shimadzu, RF-6000(Japan) was mainly used.pH meter, Model Proline B21, Becthai Equipment & Chemical (Thailand), analytical balance, Model BSA224S-CW, Scientific Promotion (Thailand), quartz cell, Fisher Scientific (USA) were also used. Round bottom flask, Pyrex^® (England) and hot plate with magnetic stirrer in association with paraffin oil bath were also needed for this experiment.

Preparation of Gqds by Thermolysis of Citric Acid

Effect of Weight of Citric Acid

Since thermolysis of solid citric acid depends on its starting weight and preheated temperature with short synthetic time, various weights of citric acid (0.25, 0.5 and 0.75 g) were used to fuse in round bottle flask which was equilibrated at 200 °C in preheated paraffin oil bath for 5 min. Then, the melted citric acid was solubilized with 50 mL of 0.25 M NaOH solution while continuous magnetic stirring for 30 min at room temperature⁹. The obtained GQDs solution was diluted to a series of 10-50 mg/L, based on the original weight of citric acid used, prior to fluorescence measurement with excitation and emission wavelengths of 365 and 460 nm, respectively. Their fluorescence intensities were plotted with respect to the GQDs concentrations, giving calibration curves with different slopes.

Effect of Heating Temperature using Paraffin oil Bath

Generally, thermolysis process is required an appropriate heating temperature in accordance with heating time due to such colligative properties of substance. Thus, in this case, the preheating temperature at 200, 225 and 250 °C was approximately (10°C more or less) set up using the paraffin oil bath using 0.25 g citric acid under the same synthetic route and fluorescence measurement as mentioned above.

Effect of Solution pH

The 1.0 M acetate buffer solution was prepared and adjusted to appropriate pH 2-5 with HCl or NaOH solutions using pH meter. 1.0 M phosphate buffer solution was prepared and also adjusted to pH 6-13in the same manner.Then, 100 mg/L GQDs solution was subject to buffering capacity at pH range of 2-13 with 1.0 mL of those buffering solutions in 10.0 mL volumetric flask prior to fluorescence measurement.

Effects of NaCl and EDTA Concentrations

One mL of various concentrations of 0.05, 0.10, 0.25, 0.20 and 0.25 M NaCl was added into 100 mg/L GQDs solution and adjusted to 10 mL in volumetric flask prior to fluorescence measurement. In the same manner, 1.0 mL of 0.01, 0.02, 0.03, 0.04and 0.05 M EDTA was added into the GQDs solution and recorded their fluorescence spectra.

Effects of Anionic salts (1% w/w) Doping on GQDs

Following the suitable synthetic procedure of GQDs as mentioned above, some representative mineral acids (HCl, HClO₄, HNO₃, H₂SO₄, H₃PO₄, H₃BO₃), of 1% w/w were used and simultaneously added into the melted citric acid within 5 min during thermolysis step, then solubilized in 50 mL of 0.25 M NaOH in brown capped glass bottle with continuous magnetic stirring for 30 min at room temperature prior to fluorescence measurement using 100 mg/L of the mineral acids doped GQDs solution.

Selectivity Test of the Mineral Acids Doped GQDs on Various Metal Ions

One mL of 10 µM each solution of Na⁺, Mg²⁺,Ag⁺, Cu²⁺, Pb²⁺, Mn²⁺, Hg²⁺,Fe²⁺, Fe³⁺and Al³⁺ was added into each of various mineral acids doped GQDs solution (100 mg/L) and adjusted final volume to 10 mL with deionized water prior to measurement of fluorescence spectrum.

Results and Discussion

Synthesis and Fluorescence Characteristics Of GQDs

Graphene quantum dots (GQDs) were synthesized via thermolysis of solid citric acid using pre-heated paraffin oil bath for 5 min after its fusion, and were solubilized with 50 mL of 0.25 M NaOH. Citric acid weights of 0.25, 0.50 and 0.75 g were subject to melt in the pre-heated oil bath at 200°C for 5 min following the procedure mentioned above. Each of stock GQDs solutions were diluted approximately ranging between 10-50 mg/L. The fluorescence spectrum of GQDs solution was recorded between 400-600 nm showing emission l_max 460 nm with excitation l_max 365 nm (data not shown). Their calibration curves were constructed comparatively as shown in Fig. 1. It was found that the calibration curve: y = 179.41x – 11.43 with highest slope, even it was slightly larger standard deviations compared to others, was obtained from using 0.25 g citric acid. While those of calibration curves gave rather lower probably due to massive dependence on short thermolysis of citric acid at 200°C within 5 min. The effect of heating temperature (200, 225 and 250°C) of paraffin oil bath on calibration curve of GQDs solution was also investigated in the same manner using0.25 g citric acid as shown in Fig. 2.It was found that both slopes of two calibration curves (y = 213.06x – 263.32 and y = 227.62x – 400.42) showed no difference and were higher than that of the other one, considerably indicating that thermolysis of 0.25 g citric acid at heating temperature between 200 °C and 225 °C was comparable, while at 250°C it is too hot for rapid transformation of GQDs.

Figure 1: Effect of weight of citric acidon calibration curve of GQDs solution (Thermolysis conditions: 0.25, 0.5 and 0.75 g citric acid, heating at 200°C for 5 min in 50 mL 0.25 M NaOH) Click here to View figure

 

Figure 2:Effect of heating temperature of paraffin oil bath on calibration curve of GQDs solution (Thermolysis conditions: 0.25 g citric acid, heating at 200, 225 and 250°C for 5 min in 50 mL of 0.25 M NaOH) Click here to View figure

 

Effects of pH, Ionic Strength and Masking Agent on Fluorescence Intensity of GQDs

Since thermolysis of citric acid to prepare GQDs was processed in atmosphere pressure, the chemical transformation of GQDs had been possessed with common functional groups like hydroxyl, epoxy and carboxyl parts. So, after solubilization of GQDs in 0.25 M NaOH, its fluorescence spectrum could be indicated. However, when GQDs were buffered with 0.1 M acetate buffer solution pH 2-5, there was no fluorescence intensity at all, acidic pH quenching effect pronounced. If GQDs were prepared in neutral to base buffer solution pH 6-13, their fluorescence intensities were recovered almost constantly as shown in Fig. 3.

In addition, effects of ionic strength as 0.05 0.10 0.15 0.20 and 0.25 M NaCl solutions and masking agent as 0.01, 0.02, 0.03, 0.04 and 0.05 EDTA solutions tested were also investigated (data not shown). It was found that both ionic strength and masking agent had no effect on the fluorescence intensity of the GQDs solution. It is further aimed that modification of the ionic strength of the mixture solution could be done in association with addition of EDTA to mask any metal ions interferences in sample.

Figure 3: Effect of solution pH on fluorescence intensity of GQDs solution (100 mg/L) Click here to View figure

 

Effect of Anionic Salts Decorating on GQDs During Thermolysis of Citric Acid

Basically, an element doping on GQDs during simple thermolysis of citric acid, using paraffin oil bath at 225 °C for 5 min and solubilized in 50 mL 0.25 M NaOH, could achieved as multi-functional carbon based materials for tuning fluorescence turn-on/off sensor probe for chemical detection. In this study, six kinds of mineral acids including hydrochloric acid, sulfuric acid, nitric acid, orthophosphoric acid, perchloric acid, and boric acid were introduced to the synthesis of GQDs. As shown in Fig. 4, two groups of fluorescence spectra were found, indicating that these anionic decorated GQDs solutions had different photoluminescence after excited at the same maximum excitation wavelength of 365 nm, resulted nearly in the same maximum emission wavelength of 460 nm. This means that these decorated GQDs exhibit simply the same fluorescence mechanism compared to that of undecorated GQDs. The enhancing effects of fluorescence intensity of the element decorated GQDs were found with three kinds of the anionic salts like Cl^–, NO₃^– and PO₄^3-, while their quenching effects were found with thermolysis of ClO₄^–, SO₄^2-, and BO₃^3-. Thus, trends in useful information for fluorescence turn-on/off sensor probe can be drawn from these results, if it is needed for various metal ions determination in any sample.

Figure 4: Fluorescence spectra of undecorated GQDs and GQDs decorated with six mineral acids (1%, w/w) solutions Click here to View figure

 

Figure 5: Quenching and enhancing effects of metal ions on fluorescence intensity of undecorated GQDs solution Click here to View figure

 

From Fig. 5, the fluorescence intensity of undecorated GQDs solution was normally obtained.⁹ When 1.0 mL each of 10 µM of Na⁺, Mg²⁺, Ag⁺, Cu²⁺, Pb²⁺, Mn²⁺, Hg²⁺, Fe²⁺, Fe³⁺ and Al³⁺ was mixed with the GQDs 100 mg/L in final volume of 10 mL, the quenching effects of Hg²⁺ and Mn²⁺were quite strong. But only Cu²⁺ gave slightly enhancing one, while other cations did not show much change in their quenching effects. So, selective and sensitivity test for Hg²⁺ and Mn²⁺ can be done, while optimization conditions for the sensitivity test for fluorescence turn-on sensor of Cu²⁺ concentration would be further investigated.

Figure 6: Quenching and enhancing effects of metal ions on fluorescence intensity of phosphate decorated GQDs solution  Click here to View figure

 

From Fig. 6, the fluorescence intensity of phosphate decorated GQDs solution was increased higher than that of undecorated GQDs solution. When 1.0 mL each of 10 µM of Na⁺, Mg²⁺, Ag⁺, Cu²⁺, Pb²⁺, Mn²⁺, Hg²⁺, Fe²⁺, Fe³⁺ and Al³⁺ was mixed with the phosphate – GQDs 100 mg/L in final volume of 10 mL, the quenching effects of Hg²⁺ and Mn²⁺ were strongly comparable, while Fe²⁺ and Al³⁺ gave pronouncedly enhancing one.Other cations did not show much change in their quenching effects compared with that of blank (phosphate – GQDs without cation) solution. Optimization conditions for the selectivity and sensitivity test for fluorescence turn-off sensor for Hg²⁺ and Mn²⁺concentrations would be further checked. In addition, that for fluorescence turn-on mode for Fe²⁺ and Al³⁺ could be also investigated for further analysis.

Figure 7: Quenching and enhancing effects of metal ions on fluorescence intensity of nitrate decorated GQDs solution Click here to View figure

 

From Fig. 7, the fluorescence intensity of nitrate decorated GQDs solution was drastically increased higher than that of undecorated GQDs solution, giving the same pattern as found in Fig. 9. When 1.0 mL each of 10 µM of Na⁺, Mg²⁺, Ag⁺, Cu²⁺, Pb²⁺, Mn²⁺, Hg²⁺, Fe²⁺, Fe³⁺ and Al³⁺ was mixed with the nitrate – GQDs 100 mg/L in final volume of 10 mL, the quenching effects of Hg²⁺ was strongly obtained. The second one was Mn²⁺ which gave slightly quenching one, while other cations showed no change in their fluorescence intensities, except Cu²⁺ showed the enhancing effect drastically compared with that of blank (nitrate – GQDs without cation) solution. Optimization conditions for the selectivity 2and sensitivity test of Hg²⁺and Mn²⁺concentration would be further studied. In addition, selectivity test for Cu²⁺ using the N doped GQDs could be of interesting subject.

Figure 8: Quenching and enhancing effects of metal ions on fluorescence intensity of chloride decorated GQDs solution Click here to View figure

 

From Fig. 8,the fluorescence intensity of chloride decorated GQDs solution was also higher than that of undecorated GQDs solution, giving the same pattern as found in Fig. 6 and Fig. 7. When 1.0 mL each of 10 µM of Na⁺, Mg²⁺, Ag⁺, Cu²⁺, Pb²⁺, Mn²⁺, Hg²⁺, Fe²⁺, Fe³⁺ and Al³⁺ was mixed with the chloride – GQDs 100 mg/L in final volume of 10 mL, the quenching effect of Hg²⁺ was strongly obtained. The second one was Mn²⁺ which gave slightly quenching one, while other cations showed rather changes in their fluorescence intensities compared with that of blank (chloride – GQDs without cation) solution. Optimization conditions for the selectivity and sensitivity test of Hg²⁺and Mn²⁺concentrations would be further studied. In addition, selective determination of Al³⁺ using Hg²⁺ quenched chloride – GQDs may be choice of further study.

Figure 9: Quenching and enhancing effects of metal ions on fluorescence intensity of sulfate decorated GQDs solution Click here to View figure

 

From Fig. 9, the fluorescence intensity of sulfate decorated GQDs solution was drastically less than that of undecorated GQDs solution. When 1.0 mL each of 10 µM of Na⁺, Mg²⁺, Ag⁺, Cu²⁺, Pb²⁺, Mn²⁺, Hg²⁺, Fe²⁺, Fe³⁺ and Al³⁺ was mixed with the sulfate – GQDs 100 mg/L in final volume of 10 mL, the quenching effect of Hg²⁺ was strongly obtained, while other cations showed no change in their fluorescence intensities, except Al³⁺ gave slightly an enhancing effect compared with that of blank (sulfate – GQDs without cation) solution. In this case, fluorescence turn-on mode was preferred, and Al³⁺ could exhibit slightly the turn-on mode. So, optimization conditions for the sensitivity test of Al³⁺ concentration would be further investigated.

Figure 10: Quenching and enhancing effects of metal ions on fluorescence intensity of perchlorate GQDs solution Click here to View figure

 

From Fig. 10, the fluorescence intensity of perchlorate decorated GQDs solution was drastically very less than that of undecorated GQDs solution. When 1.0 mL each of 10 µM of Na⁺, Mg²⁺, Ag⁺, Cu²⁺, Pb²⁺, Mn²⁺, Hg²⁺, Fe²⁺, Fe³⁺ and Al³⁺ was mixed with perchlorate – GQDs 100 mg/L in final volume of 10 mL, only strong quenching effect of Hg²⁺ was completely obtained but its sensitivity might be limited. However, the fluorescence turn-on mode of the perchlorate – GQDs was preferred. Notably, it could be useful sensor probe for other compounds.

Figure 11: Quenching and enhancing effects of metal ions on fluorescence intensity of borate decorated GQDs solution Click here to View figure

 

From Fig. 11, also the fluorescence intensity of borate decorated GQDs solution was drastically very less than that of undecorated GQDs solution, giving the same pattern as found in Fig. 9 and Fig. 10. When 1.0 mL each of 10 µM of Na⁺, Mg²⁺, Ag⁺, Cu²⁺, Pb²⁺, Mn²⁺, Hg²⁺, Fe²⁺, Fe³⁺ and Al³⁺ was mixed with the borate – GQDs 100 mg/L in final volume of 10 mL, the quenching effect of Hg²⁺ was strongly obtained. The second one was Al³⁺which gave slightly quenching one, while other cations showed no change in their fluorescence intensities compared with that of blank (borate – GQDs without cation) solution. Actually, fluorescence turn-on mode for this GQDs system was preferred, and Fe³⁺ could exhibit slightly the turn-on mode but it was quite low sensitivity. Optimization conditions for the sensitivity test of Fe³⁺ concentration would be further investigated.

Conclusion

The synthesis of graphene quantum dots (GQDs) by simple thermolysis of citric acid was successfully carried out. Fluorescence spectrum of the blue GQDs solution with l_max excitation of 365 nm was characterized. The fluorescence intensity at l_max emission of 460 nm was used to calibrate their uniformity. The GQDs in neutral-base solution gave high fluorescence intensity without any effect of ionic strength and masking agent tested. The effects of various anionic (1%, w/w) decorated with GQDs while thermolysis were also studied. From the results, nitrate, phosphate and chloride decorated GQDs exhibited an enhancement of the fluorescence intensity drastically, while perchlorate, borate and sulfate decorated GQDs gave their intrinsic quenching effects compared with that of bare GQDs. These anionic – decorated GQDs were subject for selectivity test with various metal ions. It was found that individual anionic decorated GDQs showed useful trends in their fluorescence turn-on/off, demonstrating that the as-prepared GQDs could be applied widely for further fluorescence sensors.

Acknoeledgements

The authors thank the Higher Education Research Promotion and National Research University Project of Thailand, Office of the Higher Education Commission, through the Food and Functional Food Research Cluster of Khon Kaen University, Materials Chemistry Research Center, Department of Chemistry and Center of Excellence for Innovation in Chemistry (PERCH-CIC), Thailand for financial support.

References

1. Ponomarenko, L.A.; Schedin, F.; Katsnelson, M.I.; Yang, R.; Hill, E.W.; Novoselov, K.S.; Geim, A.K. Science 2008, 320, 356-358.
   CrossRef
2. Baker, S.N.; Baker, G.A. Angew. Chem. Int. Ed. 2010, 49, 6726-6744.
   CrossRef
3. Zhu, S.; Tang, S.; Zhang, J.; Yang, B. Chem. Commun.2012, 48, 4527-4539.
   CrossRef
4. Li, L.L.; Wu, G.H.; Yang, G.H.; Peng, J.; Zhao, J.W.; Zhu, J.J. Nanoscale, 2013, 5, 4015-4039.
   CrossRef
5. Shen, J.; Zhu, Y.; Yang, X.; Li, C.Chem. Commun. 2012, 48, 3686-3699.
   CrossRef
6. Zhu, S.J.; Zhang, J.H.; Tang, S.J.; Qiao, C.Y.; Wang, L.; Wang, H.Y.; Liu, X.; Li, B.; Li, Y.F.; Yu, W.L.; Wang, X.F.; Sun, H.C.; Yang, B Adv. Funct. Mater.2012, 22, 4732-4740.
   CrossRef
7. Liu R.; Yang, R.; Qu, C.; Mao, H.; Hu, Y.; Li, J.; Qu, L. Sens. Actuators. B. 2017, 241, 644-651.
   CrossRef
8. Amjadi, M.; Manzoori, J.L.; Hallaj, T.J. Lumin.2014, 53, 73-78.
   CrossRef
9. Liu, J.J.; Chen Z.T.; Tang, D.S.; Wang, Y.B.; Kang, L.T.; Yao, J.N. Sens. Actuators. B. 2015, 212, 214-219.
   CrossRef
10. Zhou, C.; Jiang, W.; Via, B.K. Colloids Surf.B. 2014, 118,72-76.
    CrossRef
11. Wu, Z.; Li, W.; Chen, J.; Yu,C.  Talanta. 2014, 119, 538-543.
    CrossRef
12. Hormozi-Nezhad, M.R.; Taghipoura, M. Anal. Methods. 2016, 8, 4064-4068
    CrossRef
13. Kim, K.; Nam, Y.S.; Lee, Y.; Lee, K.B. J. Anal. Methods Chem. 2017,doi.org/10.1155/2017/3648564
    CrossRef
14. Chen, L.; Wu,C.; Du, P.; Feng, X.; Wu, P.; Cai, C. Talanta. 2017, 164, 100-109.
    CrossRef
15. Li, S.; Li, Y.; Cao, J.; Zhu, J.; Fan, L.; Li, X. Anal. Chem. 2014, 86, 10201-10207.
    CrossRef
16. Tripathy, S.K.; Woo, J.Y.; Han, C.S. Sens. Actuators.B. 2013, 181, 114-118.
    CrossRef
17. Chowdhury, A.D.; Doong, R.A. ACS Appl. Mater. Interfaces. 2016, 8, 21002-21010.
    CrossRef
18. Xuan, W.; Ruiyi, L.; Saiying, F.; Zaijun, L.; Guangli, W.; Zhiguo, G.; Junkang, L. Sens. Actuators. B. 2017, 243, 211-220.
    CrossRef

---

This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.

---

This work is licensed under a Creative Commons Attribution 4.0 International License.