Synergistic Effect of the Heterostructure of Au Nanoislands on TiO2

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Surfaces, Interfaces, and Applications

Synergistic effect of heterostructure of Au nanoislands on TiO2 nanowires for efficient ionization in laser desorption/ionization (LDI) mass spectrometry MoonJu Kim, Tae Gyeong Yun, Joo-Yoon Noh, Jong-Min Park, Min-Jung Kang, and Jae-Chul Pyun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b03386 • Publication Date (Web): 10 May 2019 Downloaded from http://pubs.acs.org on May 10, 2019

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ACS Applied Materials & Interfaces

Synergistic effect of heterostructure of Au nanoislands on TiO2 nanowires for efficient ionization in laser desorption/ionization (LDI) mass spectrometry

Moon-Ju Kima, Tae Gyeong Yuna, Joo-Yoon Noha, Jong-Min Parka, Min-Jung Kangb, Jae-Chul Pyuna,*

aDepartment

of Materials Science and Engineering, Yonsei University,

134 Shinchon-dong, Seodaemun-gu, Seoul 120-749, Korea Tel: +82 2 2123 5851, Fax: +82 2 312 5375 Email: [email protected]

bKorea

Institute of Science and Technology (KIST), Seoul, Korea

*To whom correspondence should be addressed

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ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract A combination nanostructured matrix with metal Au nanoislands and semiconductor TiO2 nanowires is presented to enhance both desorption and ionization efficiency in laser desorption/ionization (LDI) mass spectrometry. The heterostructure of Au nanoislands on TiO2 nanowires was fabricated via (1) TiO2 nanowire synthesis through a modified wetcorrosion method and (2) Au nanoisland formation through thermal annealing of a sputtered Au layer on the TiO2 nanowires. Herein, the synergistic effect of this heterostructure for highly efficient ion production was experimentally elucidated in terms of the formation of high temperature on the surface of Au and the creation of a Schottky barrier at the Au-TiO2 interface. Finally, four types of immunosuppressors were analyzed to demonstrate the improved ionization performance of the heterostructure for LDI mass spectrometry.

Keywords: Au-TiO2 heterostructure, wet corrosion, LDI mass spectrometry, thermal desorption, differential scanning calorimetry (DSC), photocatalytic ionization, Schottky heterojunction, synergistic effect

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1. Introduction Inorganic nanomaterials have been introduced as effective solid matrices in laser desorption/ionization (LDI) mass spectrometry instead of the conventional organic matrix molecules. Conventional organic matrices with conjugated backbone structures and acidic/basic functional groups are used to ionize large-molecular weight analytes by (1) absorbing an ultraviolet (UV) laser with conjugated structures and (2) protonating analyte molecules with the functional groups as proton sources.1–3 However, the organic matrix molecules are non-reproducibly fragmented by UV laser radiation, and the ionized fragments produce mass peaks at a low mass-to-charge ratio of 700 nm in size. Because LDI reaction occurs on the surface of nanomaterials, active surface area of Au nanoparticles on TiO2 nanowires is confined due to the severe aggregation of Au nanoparticles, resulting in poor LDI efficiency. In the case of the heterostructure of Au nanoislands on TiO2 nanowires synthesized via modified wet- corrosion, however, a homogeneous distribution of Au nanoislands was observed on the surface of the TiO2 nanowires, as shown in Figure 3(b). These results indicate that the active surface of the TiO2 nanowires was restricted for the Au nanoparticle aggregates bound to TiO2 nanowires compared with the homogeneously distributed Au nanoislands prepared on TiO2 nanowires.

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To compare the ionization efficiency of the heterostructure of Au nanoislands on TiO2 nanowires synthesized via wet-corrosion with that of other types of matrices, four immunosuppressors—tacrolimus, cyclosporin A, sirolimus, and everolimus—were used as model analytes. Immunosuppressors were primarily used in immunosuppressive therapy to prevent the rejection of transplanted organs or tissues and to treat autoimmune diseases. The dose of immunosuppressors should be controlled to be optimal to reduce side effects, such as immunodeficiency and disorder of metabolism. However, the immunosuppressors could be not ionized by the conventional MALDI-TOF mass spectrometry based on organic matrices such as CHCA and DHB as shown in Figure 4(a) and 4(b). These results indicate that the conventional matrices could not be used as a proton source, even after the UV laser irradiation. Au nanoparticles were also an ineffective matrix for the ionization of the immunosuppressors,

as shown in

Figure

4(c). These

results indicate

that

the

immunosuppressors could not be easily ionized by the thermal energy of the Au nanoparticles under the UV laser irradiation. When TiO2 nanowires synthesized via the wetcorrosion process were used as a matrix, the four immunosuppressors could be ionized, as shown in Figure 4(d), and the mass peaks were observed as K adducts. That is, the semiconducting TiO2 nanowires have more ionization power than metallic Au nanoparticles. These results indicate that the holes left at the valence band after the UV laser irradiation of the TiO2 nanowires could be used for the ionization of the immunosuppressors. When Au nanoparticles were prepared on TiO2 nanowires as matrices for LDI mass spectrometry, as shown in Figure 4(e), the four immunosuppressors exhibited mass peaks with enhanced intensities. Considering the size (14.7 nm) and concentration (1.56 x 1013 nanoparticle/cm3) of colloidal Au nanoparticles, the amount of Au for the heterostructure of Au nanoparticles

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on TiO2 nanowires was calculated to be around 5.00 x 10-4 mg per sample spot, which was quite comparable to that of Au for the heterostructure of Au nanoislands on TiO2 nanowires (3.03 x 10-4 mg per sample spot). The S/N ratios of the mass peaks for the Au nanoparticles prepared on TiO2 nanowires were 17.13, 19.42, 19.25, and 13.01 for tacrolimus, cyclosporin A, sirolimus, and everolimus, respectively. Compared with the TiO2 nanowires alone, the S/N ratios of the mass peaks for the Au nanoparticles on TiO2 nanowires were increased 32.14, 1.14, 1.33, and 2.88-fold for tacrolimus, cyclosporin A, sirolimus, and everolimus, respectively. These results show that the Au nanoparticles prepared on TiO2 nanowires were more effective than the TiO2 nanowires alone for the ionization of the four immunosuppressors, demonstrating that interface between Au nanoparticles and TiO2 nanowires has a positive effect on the ionization of the immuosuppressors. Moreover, when the nanoislands prepared on TiO2 nanowires were used as matrices for LDI mass spectrometry, as shown in Figure 4(f), the four immunosuppressors exhibited mass peaks with far enhanced intensities. The S/N ratio of the mass peaks for the heterostructure of Au nanoislands prepared on TiO2 nanowires was 208.51, 305.52, 342.32, and 271.61 for tacrolimus, cyclosporin A, sirolimus, and everolimus, respectively. Compared with the TiO2 nanowires alone, the S/N ratio of the mass peaks for the heterostructure of Au nanoislands prepared on TiO2 nanowires increased 391.20, 17.94, 23.58, and 60.69-fold for tacrolimus, cyclosporin A, sirolimus, and everolimus, respectively. As previously mentioned, LDI reaction occurs on the surface of nanomaterials. The active surface of the TiO2 nanowires was restricted for the Au nanoparticle aggregates bound to TiO2 nanowires compared with the homogeneously distributed Au nanoislands prepared on TiO2 nanowires, which was considered to result in poor LDI efficiency. These results indicate that heterostructure of

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Au nanoislands on TiO2 nanowires was much more effective than the TiO2 nanowires alone and the Au nanoparticles prepared on TiO2 nanowires cases for the ionization of the four immunosuppressors. This is because, as previously mentioned, Au nanoislands have larger surface area served as active sites for the ionization than Au nanoparticles. For the quantitative analysis of the four immunosuppressors—tacrolimus, cyclosporin A, sirolimus, and everolimus—Au nanoislands prepared on TiO2 nanowires, Au nanoparticles prepared on TiO2 nanowires, and TiO2 nanowires alone were used as solid matrices for LDI mass spectrometry. As shown in Figure 5(a), the heterostructure of Au nanoislands on TiO2 nanowires exhibited a far higher sensitivity over the entire concentration range of 1 ng/mL– 10 µg/mL and a far lower LOD than the Au nanoparticles prepared on TiO2 nanowires and the TiO2 nanowires alone. The Au nanoparticles on TiO2 nanowires exhibited a higher sensitivity and lower LOD for the quantitative analysis of the four immunosuppressors than the TiO2 nanowires alone when used as a matrix for LDI mass spectrometry. As shown in Figure 5(b), the quantitative analysis of the four immunosuppressors—tacrolimus, cyclosporin A, sirolimus, and everolimus—using Au nanoislands prepared on TiO2 nanowires for LDI mass spectrometry exhibited high linearity of 0.92, 0.93, 0.95, 0.95 over the whole concentration range of 1 ng/mL–10 µg/mL. These results indicate that the Au nanoislands prepared on TiO2 nanowires were effective as a matrix in LDI mass spectrometry for the quantitative analysis of the four immunosuppressors, with a high sensitivity and high linearity.

3.3 Mechanism of enhanced ion production with heterostructure of Au nanoislands on TiO2 nanowires

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The desorption/ionization efficiency of the Au nanoislands on TiO2 nanowires was observed to be far higher than those of solid matrices consisting of Au nanoparticles alone and TiO2 nanowires alone, as well as Au nanoparticles bound to TiO2 nanowires, for LDI mass spectrometry. Herein, the increased desorption/ionization efficiency resulting from the interaction between the Au nanoislands and TiO2 nanowires is attributed to (1) the photocatalytic activity related to the carrier separation caused by the Schottky junction between the TiO2 nanowires and Au nanoislands and (2) the thermal properties related to the enhancement of the heat release from the Au nanoislands, as illustrated in Scheme 1.

(a) Photocatalytic activity of heterostructure of Au nanoislands on TiO2 nanowires The photocatalytic activity of the heterostructure of Au nanoislands on TiO2 nanowires was estimated via the degradation of methylene blue under UV irradiation. Usually, the photocatalytic reaction of TiO2 nanowires with methylene blue is resulted from the production of hydroxide radicals by the reaction of water molecules with remaining holes at the valence band under UV irradiation. As shown in Figure 6(a), the chromophoric group of methylene blue was degraded by the electrophilic attack of hydroxyl radicals (•OH) during electronic reorganization under UV irradiation. Via this reaction, the sulfhydryl part (C-S+=C) was converted into a sulfoxide (C-S(=O)-C), and the aromatic heterocycle in the middle was opened.47 In this study, the relative photocatalytic efficiency of the Au nanoislands on TiO2 nanowires was compared with those of TiO2 nanowires, Au nanoislands on a Ti plate, and the heterostructure of Au nanoislands on TiO2 nanowires. The Au nanoislands on the Ti plate were produced via heat treatment of a Au layer with a thickness of 5 nm. Au nanoislands, TiO2 nanowires, and Au nanoislands on TiO2 nanowires with an

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area of 900 mm2 were separately reacted with methylene blue under continuous UV irradiation at a wavelength of 365 nm. As shown in Figure 6(b), Au nanoislands had the least impact on photocatalytic degradation of methylene blue even after 180 min. On the contrary, methylene blue with TiO2 nanowires was significantly oxidized over time, and furthermore, with heterostructure of Au nanoislands on TiO2 nanowires, much more methylene blue was degraded. These results indicated that the TiO2 semiconductor showed better photocatalytic activity under UV irradiation than Au metal. It is considered that unlike metals, semiconductors have bandgaps where electrons cannot be allowed, resulting in longer lifetime and lower recombination rate of photo-induced carriers in TiO2 semiconductor than Au metal. Usually, the charge separation and the prohibition of carrier recombination due to the Schottky barrier determine the photocatalytic activity of TiO2.48–51 In this study, the enhanced photocatalytic activity of the Au nanoislands on TiO2 nanowires was attributed to the prohibition of the carrier recombination due to the formation of a Schottky barrier at the interaction between the Au nanoislands and TiO2 nanowires. As depicted in Figure 6(c), a Schottky barrier was produced at the interface between the Au nanoislands and TiO2 nanowires owing to the difference in Fermi energy levels. Because the Fermi energy level of Au (Au work function = 5.1 eV) is lower than that of TiO2 (TiO2 work function = 4.2 eV), electrons flow from TiO2 to Au when they come into contact each other to equilibrate the Fermi levels and thereby energy bands of TiO2 are bent upward toward the interface, forming a Schottky barrier.48 Upon UV irradiation, electrons excited from the valence band to the conduction band of TiO2 flowed into Au, leaving holes in the valence band. The electrons that flowed to the Au had difficulty jumping back to the TiO2 owing

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to the Schottky barrier. As a similar work, in some previous studies, it is also reported that how hot electrons from plasmonic nanostructures could be used for the ionization of analytes, such as aspartic acid and juglone with lower LUMO energies than the energy level of SPR hot electrons.38 Thus, the photocatalytic performance of the heterostructure of Au nanoislands on TiO2 nanowires was enhanced by the prohibition of carrier recombination due to the Schottky heterojunction.

(b) Thermal properties of heterostructure of Au nanoislands on TiO2 nanowires Au nanoparticles were used as a matrix for LDI mass spectroscopy because Au has a very low specific heat capacity of 0.129 J/g∙K and a high thermal conductivity of 318 W/mK. Thus, it can effectively increase the surface temperature by absorbing the UV irradiation of the laser and transfer the thermal energy to the surface-adsorbed analytes.52 As the metals had no bandgap, the free electrons of Au absorbed the UV laser energy and easily reemitted it as thermal energy. This laser-induced thermal energy on the surface of Au nanoislands was considered to be transferred to the analytes and resulted in the desorption and ionization of analytes. The thermal properties of the nanostructures were analyzed with histidine as an indicator molecule by using DSC. As shown in Figure 7(a), the histidine started to be decomposed at 278.12 °C, and the required enthalpy for decomposition was 598.4 mJ. The temperature and enthalpy of histidine were not significantly changed after it was mixed with TiO2 nanowires. When Au nanoparticles and Au nanoislands on TiO2 nanowires were mixed with histidine, on the other hand, the decomposition temperature and the enthalpy decreased significantly. In particular, the Au nanoislands on TiO2 nanowires required a lower enthalpy

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than the Au nanoparticles. These changes indicate that the Au-based nanostructures could easily increase the temperature of surfaces with lower energy than TiO2 nanowires due to the metallic Au. These results show that Au nanoislands on TiO2 nanowires can be effectively used as a matrix for LDI mass spectrometry, absorbing the energy of the UV laser and increasing the surface temperature to desorb the ionized analytes. The performance of Au nanoparticles, TiO2 nanowires, and two types of heterostructures— Au nanoparticles on TiO2 nanowires and Au nanoislands on TiO2 nanowires—as solid matrices for LDI mass spectrometry was tested. When histidine was analyzed as a model analyte by using Au nanoparticles alone, the mass peaks of histidine were observed to be Na adducts [His+Na]+ with relatively lower intensity compared with the cases of TiO2based nanomaterials as solid matrices, as shown in Figure 7(b). When histidine was analyzed with Au nanoparticles alone and TiO2 nanowires alone as a matrix, the mass peaks of histidine were observed to be Na adducts [His+Na]+ and K adducts [His+K]+, and the S/N ratio was 55.02 and 431.81, respectively. These results demonstrate that metallic Au nanoparticles are appropriate for the desorption of analytes owing to their remarkable thermal properties, such as a low heat capacity and a high thermal conductivity, but insufficient for the ionization than semiconducting TiO2 nanowires.45 When Au nanoparticles on TiO2 nanowires were used as a solid matrix, noise peaks were observed because of the organic citrate molecules which stabilize the colloidal Au nanoparticles (Figure S5). When Au nanoislands on TiO2 nanowires were used as a matrix for LDI mass spectrometry, the histidine exhibited only two major mass peaks of [His+K]+ and [His+2K]+ with reduced noises. Because the Au nanoislands on TiO2 nanowires were produced without organic capping agents and thermally annealed at 600 ℃, there were no organic

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impurities which could interfere the mass analysis. Additionally, the S/N ratio of the mass peaks increased 1.17-fold compared with the cases of Au nanoparticles on TiO2 nanowires as solid matrix. These results show that Au nanoislands on TiO2 nanowires can be effectively used as a matrix for LDI mass spectrometry, owing to the enhanced photocatalytic and thermal properties for ionizing analytes and desorbing the ionized analytes. That is, the metallic Au nanoislands with outstanding thermal properties and the semiconducting TiO2 nanowires with good photocatalytic activities are beneficial to the desorption and ionization of analytes, respectively. In addition, the Schottky heterojunction between Au nanoislands and TiO2 nanowires prevents the charge carrier recombination, leading to the enhancement

of

photocatalytic

efficiency.

Hence,

the

synergistic

effect

of

the

heterostructure of Au nanoislands on TiO2 nanowires was confirmed.

4. Conclusions A heterostructure that has a metal nanostructure of Au nanoislands and semiconductor TiO2 nanowires is designed for the enhancement of ion production in LDI mass spectrometry. Since the desorption and ionization processes are coupled together and occur within a very short time, it’s difficult to distinguish these two contributions on LDI efficiency explicitly. In this work, the individual impact of the heterostructure on the desorption and ionization steps was elucidated in aspects of the thermal and electrical properties of Au-TiO2 heterostructure. The heterostructure was fabricated via two different methods: (1) the synthesis of TiO2 nanowires through a modified wet-corrosion method and (2) the formation of Au nanoislands via thermal annealing of a sputtered Au layer. The correlation between the size of the Au nanoislands and the ionization efficiency for

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immunosuppressors was investigated using LDI mass spectrometry, and the optimal size of the Au nanoislands was achieved by using a Au layer with a thickness of 5 nm. To compare the ionization efficiency of the Au nanoislands on TiO2 nanowires synthesized via wet-corrosion with that of other types of matrices, four types of immunosuppressors— tacrolimus, cyclosporin A, sirolimus, and everolimus—were used as model analytes. The heterostructure of Au nanoislands on TiO2 nanowires most effectively ionized the four immunosuppressors, in comparison with the TiO2 nanowires alone and the Au nanoparticles on TiO2 nanowires. Finally, to elicit the individual impact of the heterostructure on the desorption and ionization, the thermal behaviors and photocatalytic activities of the heterostructure were analyzed using oxidation reaction of methylene blue and DSC, respectively. The increased desorption/ionization efficiency of the heterostructure was attributed to (1) the enhanced photocatalytic activity related to the carrier separation caused by the Schottky junction between the TiO2 nanowires and Au nanoislands and (2) the high surface temperature on Au nanoislands owing to the low heat capacity of Au.

Acknowledgements This work was supported by the Nano-Convergence Foundation [grant number: R201602210] funded by the Ministry of Science, ICT and Future Planning (MSIP, Korea) and the Ministry of Trade, Industry and Energy (MOTIE, Korea); the Industry Technology Development Program [grant number: 10063335] funded by the Ministry of Trade, Industry and Energy (MOTIE, Korea); and the National Research Foundation of Korea [grant numbers: NRF-2017R1A2B4004077, NRF-2017R1A2B2004398].

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Figure legends

Scheme 1. Schematic illustration of the synergistic LDI performance in the heterostructure of Au nanoislands and TiO2 nanowires.

Figure 1. Synthesis of a combination matrix of Au nanoislands on TiO2 nanowires. (a) Schematic illustration of synthesis method for a heterostructure of Au nanoislands on TiO2 nanowires. (b) Structural mechanism for the formation of TiO2 nanowires via the whole wet-corrosion method.

Figure 2. Thickness optimization of the as-deposited Au nanofilm on TiO2 nanowires for LDI-TOF mass spectrometry using the immunosuppressive drugs including tacrolimus, cyclosporin A, sirolimus, and everolimus as analytes. (a) Relationship between the thickness of the Au nanofilm and the size of the Au nanoislands synthesized on the TiO2 nanowires. (b) LDI-TOF mass spectra of four immunosuppressors with Au nanoislands on TiO2 nanowires according to the different thicknesses of the Au nanofilm.

Figure 3. Comparison of the spatial distribution of Au nanoparticles and Au nanoislands on TiO2 nanowires. (a) Inhomogeneous distribution of Au nanoparticles bound to TiO2 nanowires. (b) Homogeneous distribution of Au nanoislands synthesized on TiO2 nanowires.

Figure 4. Comparison of the mass spectra of the four immunosuppressive drugs with the organic matrices: (a) CHCA and (b) DHB and solid matrices: (c) Au nanoparticles, (d) TiO2

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nanowires, (e) Au nanoparticles on TiO2 nanowires, and (f) Au nanoislands on TiO2 nanowires.

Figure 5. Quantitative mass analyses of the four immunosuppressors with solid matrices of TiO2 nanowires, Au nanoparticles on TiO2 nanowires, and Au nanoislands on TiO2 nanowires. (a) Comparison of the standard curves of the four immunosuppressors with TiO2 nanowires, Au nanoparticles on TiO2 nanowires, and Au nanoislands on TiO2 nanowires. (b) Sensitive quantification of the four immunosuppressors with high linearity using Au nanoislands on TiO2 nanowires as solid matrix.

Figure 6. Photocatalytic activity of the Au nanoislands on TiO2 nanowires heterostructure. (a) Photocatalytic degradation mechanism of methylene blue in a heterosturcture of Au nanoislands on TiO2 nanowires. (b) Comparison of the photocatalytic activities of the Au nanoislands, TiO2 nanowires, and Au nanoislands on TiO2 nanowires based on the methylene blue degradation. (c) Formation of a Schottky heterojunction between the Au nanoislands and TiO2 nanowires and enhancement of photocatalytic efficiency of the heterostructure of Au nanoislands on TiO2 nanowires.

Figure 7. Desorption and ionization of histidine with the nanostructured materials consisting of Au metal and TiO2 semiconductor. (a) DSC analysis of histidine mixed with solid matrices of TiO2 nanowires, Au nanoparticles, and Au nanoislands on TiO2 nanowires. (b) LDI-TOF mass analysis of histidine with solid matrices of Au nanoparticles, TiO2 nanowires, Au nanoparticles on TiO2 nanowires, and Au nanoislands on TiO2 nanowires.

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Scheme 1. Schematic illustration of the synergistic LDI performance in the heterostructure of Au nanoislands and TiO2 nanowires.

33



Figure 1. Synthesis of a combination matrix of Au nanoislands on TiO2 nanowires. (a) Schematic illustration of synthesis method for a heterostructure of Au nanoislands on TiO2 nanowires.

(b) Structural mechanism for the formation of TiO2 nanowires via the whole wet-corrosion method.

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Figure 2. Thickness optimization of the as-deposited Au nanofilm on TiO2 nanowires for LDI-TOF mass spectrometry using the immunosuppressive drugs including tacrolimus, cyclosporin A, sirolimus, and everolimus as analytes.

(a) Relationship between the thickness of the Au nanofilm and the size of the Au nanoislands synthesized on the TiO2 nanowires.

(b) LDI-TOF mass spectra of four immunosuppressors with Au nanoislands on TiO2 nanowires according to the different thicknesses of the Au nanofilm.

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Figure 3. Comparison of the spatial distribution of Au nanoparticles and Au nanoislands on TiO2 nanowires. (a) Inhomogeneous distribution of Au nanoparticles bound to TiO2 nanowires.

(b) Homogeneous distribution of Au nanoislands synthesized on TiO2 nanowires.

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Figure 4. Comparison of the mass spectra of the four immunosuppressive drugs with the organic matrices: (a) CHCA and (b) DHB and solid matrices: (c) Au nanoparticles, (d) TiO2 nanowires, (e) Au nanoparticles on TiO2 nanowires, and (f) Au nanoislands on TiO2 nanowires.

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Figure 5. Quantitative mass analyses of the four immunosuppressors with solid matrices of TiO2 nanowires, Au nanoparticles on TiO2 nanowires, and Au nanoislands on TiO2 nanowires.

(a) Comparison of the standard curves of the four immunosuppressors with TiO2 nanowires, Au nanoparticles on TiO2 nanowires, and Au nanoislands on TiO2 nanowires.

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(b) Sensitive quantification of the four immunosuppressors with high linearity using Au nanoislands on TiO2 nanowires as solid matrix.

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Figure 6. Photocatalytic activity of the heterostructure of Au nanoislands on TiO2 nanowires.

(a) Photocatalytic degradation mechanism of methylene blue in a heterostructure of Au nanoislands on TiO2 nanowires.

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(b) Comparison of the photocatalytic activities of the Au nanoislands, TiO2 nanowires, and Au nanoislands on TiO2 nanowires based on the methylene blue degradation.

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(c) Formation of a Schottky heterojunction between the Au nanoislands and TiO2 nanowires and enhancement of photocatalytic efficiency of the heterostructure of Au nanoislands on TiO2 nanowires.

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Figure 7. Desorption and ionization of histidine with the nanostructured materials consisting of Au metal and TiO2 semiconductor. (a) DSC analysis of histidine mixed with solid matrices of TiO2 nanowires, Au nanoparticles, and Au nanoislands on TiO2 nanowires.

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(b) LDI-TOF mass analysis of histidine with solid matrices of Au nanoparticles, TiO2 nanowires, Au nanoparticles on TiO2 nanowires, and Au nanoislands on TiO2 nanowires.

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Synergistic Effect of the Heterostructure of Au Nanoislands on TiO2

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