Enhancement of Image Contrast, Stability, and SALDI-MS Detection

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Enhancement of Image Contrast, Stability, and SALDIMS Detection Sensitivity for Latent Fingerprint Analysis by Tuning the Composition of Silver-Gold Nanoalloys Yu-Hong Cheng, Yue Zhang, Siu-Leung Chau, Samuel Kin-Man Lai, Ho-Wai Tang, and Kwan-Ming Ng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09668 • Publication Date (Web): 17 Oct 2016 Downloaded from http://pubs.acs.org on October 23, 2016

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ACS Applied Materials & Interfaces

Enhancement of Image Contrast, Stability, and SALDI-MS Detection Sensitivity for Latent Fingerprint Analysis by Tuning the Composition of Silver-Gold Nanoalloys Yu-Hong Cheng, Yue Zhang, Siu-Leung Chau, Samuel Kin-Man Lai, Ho-Wai Tang, and Kwan-Ming Ng* Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, People’s Republic of China.

ABSTRACT: Metal alloy nanoparticles (NPs) offer a new combination of unique physicochemical properties based on their pure counterparts, which can facilitate the development of novel analytical methods. Here, we demonstrated that Ag-Au alloy NPs could be utilized for optical and mass spectrometric imaging of latent fingerprints (LFPs) with improved image contrast, stability, and detection sensitivity. Upon deposition of Ag-Au alloy NPs (Ag:Au = 60:40% wt), ridge regions of the LFP became amber-coloured, while the groove regions appeared purple-blue. The presence of Au in the Ag-Au alloy NPs suppressed aggregation behaviour compared to pure AgNPs, thus improving the stability of the developed LFP images. In addition, the Ag component in the Ag-Au alloy NPs enhanced optical absorption efficiency compared to pure AuNPs, resulting in higher contrast LFP images. Moreover, varying the Ag-Au ratio could enable the tuning of the resulting surface plasmonic resonance absorption and hence affect image contrast. Furthermore, the Ag-Au alloy NPs assisted the Surface-Assisted Laser Desorption/Ionization MS analysis of chemical and biochemical compounds in LFP, with better detection sensitivity than either pure AgNPs or AuNPs.

KEYWORDS: silver-gold nanoparticles, nanoalloys, latent fingerprints, image contrast, image stability, SALDI-MS

1. INTRODUCTION Nanomaterials have been commonly used in different fields of analytical sciences, such as chemical, biological, and medical analysis.1-5 Metal-based nanoparticles (NPs) are a common type of nanomaterial, which possesses exceptional physical and chemical properties (e.g., optical, electrical, thermal, and catalytic, etc), mostly due to its high surface area-to-volume ratio, specific surface adsorption, and size- and shape-dependent behaviour.6-8 Metal NPs have been widely utilized in analytical applications, such as microextraction of selected analytes by TiO2

NPs,9-10 bio-sensing by AuNPs (via Surface Plasmon Resonance and Surface Enhanced Raman Spectroscopy)11-12 and chromatographic separation using AuNPs with specialized surface functionalization as the stationary phase.13-14 In addition, NPs of noble metals such as Ag,15-19 Au,20-21 and Pt22 have also been developed as a laser-absorbing substrate for Surface-Assisted Laser Desorption/Ionization (SALDI) for small molecule analysis. Although the physicochemical properties of metal NPs can be tuned by changing their size and shape, the property variations of a pure element are still

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somewhat limited, which may restrict its applications. Two common methods have been adopted to modify metal NPs in order to enrich their properties for new analytical applications. First, surface modifications with organic ligands and antibodies have been reported for AuNPs, in order to enhance binding specificity and enrich sensing applications.23 Another method comprises the hybridization of two or more metals, forming metal alloy NPs. For instance, Fe and Co are commonly introduced into other metal NPs to add magnetic susceptibility for magnetic separation/enrichment.24 In addition, Au-Ag-Au core@shell NPs have been synthesized as electrochemical immunosensors due to their excellent electrocatalytic activity towards the reduction of hydrogen peroxide.25 Although the above metal NP modifications and hybridizations often require complicated/time-consuming synthetic procedures, the number of related studies continues to increase because proper modification/hybridization would enhance or even introduce new properties into the resulting NPs for novel analytical applications. The detection of latent fingerprints (LFPs) is critical for forensic analysis. LFP analysis nowadays not only aims at the visualization of ridge and groove patterns, but also targets recovering as much chemical (e.g., illicit compounds, explosives, etc.) and biochemical (e.g., endogenous metabolites, DNA, etc.) information as possible.26-28 Hence, new analytical technologies have been applied to LFPs detection to meet the increasing needs of forensic analysis. Ag has long been used for LFP detection in the physical developer method, in which colloidal Ag (I) ions are reduced to Ag on the lipid residue of ridges and reveal the LFP.29 However, this is a wet method which may not be suitable for fragile or water-sensitive evidence. Recently, AuNPs have been developed for assisting the double imaging of latent fingerprints via direct optical visualization and mass spectrometric imaging, in a solvent-free manner by argon

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ion sputtering.30-31 In fact, AgNPs have higher optical absorption efficiency in the visible region than AuNPs,32 which help to develop bright yellow ridges in LFP images, while AuNPs could develop pink ridges. However, the chemical stability of AgNPs is lower than that of AuNPs, and AgNPs are more susceptible to environmental changes.33 These factors could limit the stability of the LFP images. Here, we developed Ag-Au alloy NPs for the visualization of LFP, which provides a good balance of optical absorption efficiency (i.e., image clarity) and image stability. Moreover, the colour of LFP ridges could be tuned from yellow, amber, pale orange, to pink by increasing the Au content in Ag-Au alloy NPs. Using argon ion sputtering as a convenient method for the preparation of NPs, Ag-Au alloy NPs of different Ag-Au compositions and particle sizes can be coated on LFPs without employing wet chemical procedures. It is expected that LFP images developed by Ag-Au alloy NPs would have better image clarity, contrast, and stability than those developed by either pure AgNPs or AuNPs. Furthermore, the deposited Ag-Au alloy NPs can serve as a laser-absorbing substrate for the SALDI-MS detection of endogenous metabolites (and other exogenous substances, if any) in the LFP. The improved optical absorption could result in stronger photothermal heating, and thus is expected to enhance SALDI-MS detection sensitivity.34 In short, the utilization of Ag-Au alloy could combine the advantages of AgNPs and AuNPs for the improvement of image contrast, stability, and SALDI-MS detection sensitivity for LFP analysis.

2. EXPERIMENTAL SECTION In the preparation of fingerprint samples for optical image development, the thumb was wiped on the forehead and pressed on the desired substrate (glass or paper). The fingerprint images were developed by argon ion sputtering using a sputter coater (SCD 005; Bal-Tec AG, Liechtenstein) equipped with one of the following six targets: pure 2


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gold, pure silver, and silver-gold alloy (20-80% wt, 40-60% wt, 60-40% wt, and 80-20% wt). For the evaluation of image stability, developed LFP samples were kept in disposable Petri dishes. All these dishes were stored in the laboratory under ambient room conditions. In the preparation of samples for MS detection sensitivity studies, instead of wiping the thumb on the forehead, the thumb was applied with a small amount of standard mixture of palmitic acid, stearic acid, and oleic acid. To record the visible absorption spectra of the nanoparticles in the ridge region, a thin layer of sebum was smeared on a quartz slide before sputtering. In addition, a clean blank quartz slide was used for sputtering for the measurement of the visible spectra of the nanoparticles in the groove region. UV-visible absorption spectra were recorded using a UV-vis spectrophotometer (Cary 60; Agilent Technologies, Santa Clara, CA, USA). For Transmission Electron Microscopic (TEM) examination, the latent fingerprint was blotted onto a TEM grid and examined under a scanning transmission electron microscope (Tecnai G2 20 S-TWIN; FEI, Hillsboro, OR, USA). To analyze the nanoparticles “near ridge edge”, the ride edge was firstly located under the fluorescent screen with a lower magnification power (~500X), where the groove region appeared as green colour but the ridge region appeared as grey colour (due to the presence of grease). Then, the nanoparticles near the ridge edge were examined under a high magnification power (~100000X). All mass spectrometric experiments were performed using a Bruker Daltonics Ultraflex II MALDI TOF/TOF equipped with a 355nm Nd:YAG solid state smartbeam I laser, and operated in negative reflectron mode. The experimental details are provided in Supporting Information.

3. RESULTS AND DISCUSSION 3.1. Image Contrast. In this study, we applied Ag-Au alloy NPs for the visualization of latent fingerprints. Although LFPs could be developed by the deposition of pure AgNPs or AuNPs, the use of Ag-Au alloy NPs achieved

a better compromise between colour intensity and stability. In general, LFPs developed by Ag-Au alloy NPs were light yellow/orange in the ridge region and violet/grey-blue in the groove region (Figure 1a). By gradually increasing the Au content in NPs (from 0, 20, 40, 60, 80, 100% wt), the surface plasmon resonance (SPR) extinction maximum located at 420nm (in ridges) would begin red-shifting, dropping, and finally flattened for Ag0Au100 NPs (Figure 1b). The red-shift of SPR wavelength is a widely observed plasmonic property of Ag-Au alloy NPs.33,35-38 NPs with increasing Au content appeared to be paler in the LFP ridges (Figure 1a), confirmed by the decrease in absorbance (Figure 1b). This result could be explained by the decline in extinction coefficient35 and the upsurge of damping effect on the plasmon38 with increasing Au content in Ag-Au alloy NPs. The two different colours in the ridge and groove regions were due to the distinct SPR bands of the NPs, which can be affected by their shape. The TEM images revealed that Ag60Au40 alloy NPs in the ridges were generally pseudo-spherical (⌀: < 5nm), while their counterparts in the grooves were irregular in shape and mostly > 10nm (Figure 1c). This may be due to the entrapment of NPs by the lipid residue in the LFP ridges, which suppressed the aggregation of NPs and resulted in smaller sizes. Governed by nanoparticle shape, size, and surrounding environment,39-40 the SPR band of Ag60Au40 alloy NPs in ridges was narrow and peaked at ~ 450nm (Figure 1d, brown line), while the NPs in grooves had a red-shifted, broader band across 550 – 650nm (Figure 1d, blue line). These plasmonic differences explained the colour contrast between the ridges and the grooves. In addition, the sputtering time for Ag-Au alloy NPs deposition is a critical parameter in determining the clarity of LFPs. Sputtering time has direct effect(s) on the size, density, and morphology of the NPs deposited, and thus affects the colour and contrast of the 3



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Figure 1. (a) Latent fingerprint images developed on glass coverslip by the six NP compositions (Ag100Au0, Ag80Au20, Ag60Au40, Ag40Au60, Ag20Au80, and Ag0Au100) at sputtering time 60s. (b) UV-vis spectra of the six NP compositions (Ag100Au0, Ag80Au20, Ag60Au40, Ag40Au60, Ag20Au80, and Ag0Au100) deposited on sebum smear at a sputtering time 60s. (c) TEM images of Ag60Au40 NPs on ridge and groove regions with a sputtering time 60s. (d) UV-vis spectra of Ag60Au40 NPs deposited on a sebum smear mimicking the ridge region and a blank quartz coverslip mimicking the groove regions, respectively. (e) Latent fingerprint images developed on glass coverslip by Ag60Au40 NPs with sputtering time varying from 20s to 110s. Dynamic range is boxed in black. ridge and groove regions. For LFPs developed by Ag60Au40 alloy NPs (Figure 1e), a short sputtering time (i.e. ≤ 20s) resulted in a faint and barely visible LFP image attributed to an insufficient amount of NPs, while a long sputtering time (i.e. ≥ 110s) resulted in a blurred and unevenly coloured LFP image which was due to an excessive deposition of

NPs. Within the optimal range of sputtering time (i.e., the dynamic range), LFPs could be visualized with acceptable clarity and contrast. For instance, for LFPs developed by Ag60Au40 alloy NPs with an increasing sputtering time from 30s to 100s (boxed in Figure 1e), the fingerprint images would be observed clearly with colour intensified from 4


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Table 1. General Features of Fingerprint Images Developed on Glass Substrate NP Composition (% wt) Ag100Au0 Ag80Au20 Ag60Au40 Ag40Au60 Ag20Au80 Ag0Au100

Colour on Ridge Yellow Yellow Amber Pale orange Pale orange Pink

Colour on Groove Purple Purple Purple-blue Grey-blue Grey-blue Blue

Dynamic Range (second) 40-120 40-120 30-100 30-80 30-80 30-60

Table 2. Stability of Fingerprint Images Developed on Glass Substrate NP Composition Preservation Time Initial Colour (% wt) Ag100Au0 Ag80Au20 Ag60Au40 Ag40Au60 Ag20Au80 Ag0Au100

4 days 7 days > 1 month > 1 month > 1 month > 1 month

Final Colour Ridge Groove Ridge Groove Yellow Purple Pale yellow Purple-pink Yellow Purple Pale yellow Grey-blue Violet Amber Purple-blue Pale orange Pale orange Grey-blue Orange-red Grey-blue Pale orange Grey-blue Orange-red Grey-blue Pink Blue Pink Grey-blue

yellow to amber on the ridges, and from pale blue to blue on the grooves. As summarized in Table 1, higher Ag content NPs led to a wider dynamic range. The optical images showing the dynamic time range of the LFPs developed on both glass and paper substrates by different alloy NPs are also provided in Figure S1 and S2, Supporting Information. 3.2. Image Stability. LFP images with a long preservation time are desirable for repeated measurements and verification. As shown in Figure 2a, the LFP developed by the Ag100Au0 NPs (i.e., pure Ag NPs) was unstable and the image degraded significantly after 4 days with a colour change from bright yellow to very pale yellow in ridges and from purple to dull purple-pink in grooves. Moreover, in the central part of the LFP image, the fringe patterns merged together, and the ridges and grooves became indistinguishable. When the Au content in the Ag-Au alloy increased from 0% wt to 20% and ≥40%, the preservation time was extended from 4 days to 7 days and over 34

days (no colour fading or ridge merging was observed), respectively. The colour change and the preservation time of the LFP images on the glass substrate are summarized in Table 2. In addition to optical images, the UV-vis absorption of Ag-Au alloy NPs (deposited on the LFP lipid residue) was also monitored (Figure S3, Supporting Information). The changes in UV-vis absorption of the Ag-Au alloy NPs were in good agreement with the stability of the developed LFP images (Figure 2a). For instance, the optical absorption peak of Ag100Au0 NPs gradually decreased by 50% after 4 days (fading of the LFP image), and became almost flattened after 7 days (Figure 2bi). On the other hand, the change of the SPR band of Ag60Au40 alloy NPs was minor after 30 days (Figure 2bii), supporting good image stability. The stabilizing effect was more obvious when Ag0Au100 NPs was used (Figure 2biii). For the LFP images developed on the paper substrate, the preservation time was generally shorter than that of glass. Ag20Au80 and Ag0Au100 produced the most 5



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Figure 2. (a) Initial fingerprint images (left half) and the corresponding preserved images (right half) developed with the six NP compositions (i) Ag100Au0, (ii) Ag80Au20, (iii) Ag60Au40, (iv) Ag40Au60, (v) Ag20Au80, and (vi) Ag0Au100. (b) (c) TEM image showing the aggregation of Ag100Au0 and Ag60Au40 NPs at the near ridge edge on the 0th, 3rd, and 7th day after sputtering. (d) Theoretical calculation of stabilization energy per atom gained by aggregation of a pair of NPs for the six NP compositions (i.e., Ag100Au0, Ag80Au20, Ag60Au40, Ag40Au60, Ag20Au80, Ag0Au100). stable LFP images on paper substrate (stable > paper because it has the best compromise 1 month), yet Ag60Au40 was still considered between optical contrast and stability (stable as the most suitable for revealing the LFP on for 12 days), while Ag20Au80 and Ag0Au100 6 ACS Paragon Plus Environment

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only produced relatively pale images (Table LFP upon laser irradiation.27,41 Based on S1, Supporting Information). accurate mass measurement, the identity of these eight fatty acids were assigned (Table Aggregation of NPs at the ridge/groove S2, Supporting Information). MS images of interface may cause merging of fringe LFPs could be generated from the spatial patterns on the LFP image developed by distribution of the selected deprotonated fatty Ag100Au0 NPs (i.e., pure AgNPs) and acids, as shown in Figure 3b. Apart from the Ag80Au20 alloy NPs. For comparison fatty acids, other ion peaks were also recorded purpose, Ag100Au0 NPs and Ag60Au40 in the mass spectrum (Figure S6, Supporting alloy NPs near the edge of the ridge were Information). These ion peaks could be monitored by TEM on the 0th, 3rd, and 7th day originated from other endogenous metabolites after sputtering (Figure 2c). Ag100Au0 NPs secreted on the skin, and/or exogenous near the ridge edge were small and uniform substances (e.g. environmental contaminants, (~3 nm in size) immediately after sputtering. personal care products) on the skin surface, After 3 days and 7 days, some of the NPs though they were not identified in the current aggregated to form larger NPs with ~ 5 nm study. and > 10 nm, respectively. Moreover, the size distribution became much wider, indicating Ag-Au alloy NPs SALDI substrate generally the aggregation of Ag100Au0 NPs. When the provided better detection sensitivity than Au content was increased to 40% wt (i.e., either Ag100Au0 NPs or Ag0Au100 NPs. Ag60Au40), the size of NPs did not deviate Figure 3c shows that the SALDI-MS significantly from the initial size and no detection sensitivities for three selected significant aggregation was observed after 7 deprotonated fatty acids increased with Ag days. This revealed that increasing Au content content in the Ag-Au alloy NP (from 0% to could slow down the aggregation process 40% wt of Ag), and then decreased when the (Figure S4, Supporting Information). Based Ag content increased beyond 40% wt. Similar on theoretical calculation, aggregation of trends were also observed at several fixed Ag-Au alloy NPs with a naked surface is sputtering times, including 90s, 150s, and thermodynamically favorable. The simulation 300s, suggesting that the difference was results revealed that Ag100Au0 NPs exhibited probably due to the alloy compositions the largest gain in stabilization energy after (Figure S5, Supporting Information) which aggregation (Figure 2d). Furthermore, affected the SALDI substrate increasing Au content in the Ag-Au alloy NPs physicochemical properties, and thus the would impede the aggregation process, as analytical performance. revealed by the gradual decrease of stabilization energy gained via NP SALDI involves a series of complicated aggregation. Hence, the stability of the colour processes, including optical absorption, and clarity of LFP images could be enhanced thermoelectric effect, desorption, ionization, by increasing Au content in the alloy NPs. etc., which can be affected by various physicochemical properties of the SALDI 3.3. SALDI-MS Detection Sensitivity. substrate.34,42-45 Hence, a good SALDI Apart from direct optical visualization of performance would be the result of a balanced LFPs, the Ag-Au alloy NPs could also serve interplay between different physicochemical as a laser-absorbing substrate for SALDI-MS properties of the substrate. For instance, a low detection of endogenous compounds in the melting point SALDI substrate could facilitate LFPs. As shown in Figure 3a, eight previously phase transition processes.32,43-44 Since Ag reported deprotonated fatty acids (at m/z 227, NPs possess a lower melting point than Au 241, 253, 255, 269, 279, 281, and 283) were NPs,32 we anticipated that the increasing Ag detected from an Ag60Au40 alloy NPs-coated content could decrease the melting point of 7 ACS Paragon Plus Environment


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Figure 3. (a) MS spectrum showing eight deprotonated fatty acids detected upon 355nm Nd:YAG laser irradiation (laser fluence of 69.58 mJcm-2) on the Ag60Au40-coated fingerprint (sputtering time = 100s). (b) Spatial distribution of the eight deprotonated fatty acids. (c) Relationship between MS detection sensitivity and the six NP compositions (i.e., Ag100Au0, Ag80Au20, Ag60Au40, Ag40Au60, Ag20Au80, Ag0Au100) at sputtering time 90s the alloy NPs, and thus increase the ion intensity. In addition, the thermal conductivity of the substrate could be another crucial factor affecting SALDI efficiency.32,34,43,45 The thermal confinement effect could be enhanced by substrates with low thermal conductivity, resulting in a high localized laser-induced heating temperature for thermal desorption of analytes.32,43,45-46 We expected that the higher thermal conductivity of Ag NPs (27.60 Wm-1K-1)32 than that of Au NPs (16.35 Wm-1K-1)32 may reduce the thermal confinement effect of the Ag-Au alloy NPs when the Ag content increased, and thus may lower the ion intensity. Moreover, other factors, such as the effects of Ag and Au content on the optical absorption efficiencies of Ag-Au alloy NPs32,47-49 and their specific heat capacities may also exert competitive effects on the ion intensity. Hence, the resultant detection sensitivity could be

attributed to the combined effects of various physicochemical properties of the alloy NPs rather than to a single factor only. Hence, we believe that these various physicochemical properties could be balanced in Ag-Au alloy NPs to achieve an improvement in the SALDI-MS performance.

4. CONCLUSIONS This study demonstrated the use of Ag-Au alloy NPs for LFP detection via optical visualization and mass spectrometric imaging. The utilization of Ag-Au alloy NPs showed distinctive advantages over either pure AgNPs or AuNPs, in terms of a higher SALDI-MS sensitivity, and a better balance between the optical image contrast and stability. Using Ag60Au40 alloy NPs, LFPs would be developed in amber colour, against a purple-blue background with good optical 8


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contrast. LFPs on both porous (e.g. paper) and non-porous (e.g. glass) surfaces could be stable over 12 days and 1 month respectively. By using SALDI-MS, several endogenous fatty acids were detected unambiguously. Their spatial distribution could be used for tracing the chemical image of the LFP. Alloying the two noble metals allowed the combination and enrichment of their physicochemical properties, which enhanced the overall analytical performance. For instance, increasing the Au content in the alloy NPs improved chemical stability and suppressed particle aggregation, resulting in more stable LFP optical images (preservation time extended from 4 days to over 1 month). On the other hand, increasing the Ag content in the alloy NPs improved the optical absorption efficiency, which resulted in LFP images with better optical contrast. Furthermore, by balancing various physicochemical properties (e.g., decrease melting point by addition of Ag, and decrease thermal conductivity by addition of Au), Ag-Au alloy NPs generally exhibited better SALDI-MS sensitivity over either of the pure noble metal NPs. This could due to higher ion desorption efficiency via an enhanced laser-induced heating or even laser-induced phase transition/explosion process.32,50 Through this study, it has demonstrated that the use of metal alloy NPs constitutes a simple and practical method for improving analytical performance in various aspects, such as stronger optical absorption, better physical and chemical stability, and higher SALDI-MS sensitivity, over the use of pure metal NPs. It is anticipated that these advantages of the method could be beneficial to the analysis of portable samples that likely contain useful LFPs. With numerous combinations in alloy compositions and the convenient approach of argon ion sputtering, versatile types of metal alloy NPs could be prepared easily for the development and improvement of different analytical methods in the near future.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Detailed experimental procedures; Tables regarding the stability of fingerprint images developed on paper, mass-to-charge ratio of the 8 deprotonated fatty acids detected; Figures regarding the image of LFPs developed on glass and paper substrates by the six NP compositions with different sputtering time, UV-Vis spectra of the six NP compositions measured along with time, TEM images showing the suppression of the aggregation of the Ag-Au alloy NPs by increasing Au content, and effect of alloy composition on MS detection sensitivity.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel.: +(852)-2219 4696. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS We thank Mr. Frankie Y. F. Chan of the Electron Microscope Unit of The University of Hong Kong for assisting in the TEM measurements. YHCheng acknowledges The University of Hong Kong for granting the University Postgraduate Fellowship (Jessie Ho Memorial Postgraduate Fellowship). KMNg acknowledges the funding support of the General Research Fund (Grant No.: HKU_17304014) of the Hong Kong Research Grants Council.

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Enhancement of Image Contrast, Stability, and SALDI-MS Detection

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