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Fabrication of Nanostructured Mesoporous Germanium for Applications in Laser Desorption Ionization Mass Spectrometry Hazem H. Abdelmaksoud, Taryn M Guinan, and Nicolas H. Voelcker ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14362 • Publication Date (Web): 20 Jan 2017 Downloaded from http://pubs.acs.org on January 31, 2017

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ACS Applied Materials & Interfaces

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Fabrication of Nanostructured Mesoporous

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Germanium for Application in Laser Desorption

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Ionization Mass Spectrometry

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Hazem H. Abdelmaksoud1,2, Taryn M. Guinan1 and Nicolas H. Voelcker1,2*

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1.

Future Industries Institute, University of South Australia. South Australia, 5095 Mawson Lakes,

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University Boulevard 2.

ARC Centre of Excellence in Convergent Bio-Nano Science and Technology, Future Industries

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Institute, University of South Australia. *

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E-mails of corresponding authors: [email protected]

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KEYWORDS:

Germanium, Mesoporous germanium, SALDI-MS, Bipolar electrochemical

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etching, Cocaine, Illicit drugs, Laser desorption ionization.

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ACS Paragon Plus Environment

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ABSTRACT

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Surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) is a high-

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throughput analytical technique ideally suited for small molecule detection from different bodily

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fluids (e.g. saliva, urine, and blood plasma). Many SALDI-MS substrates require complex

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fabrication processes and further surface modifications. Furthermore, some substrates show

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instability upon exposure to ambient conditions and need to be kept under special inert

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conditions. We have successfully optimized mesoporous germanium (meso-pGe) using bipolar

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electrochemical etching (BEE) and efficiently applied meso-pGe as a SALDI-MS substrate for

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the detection of cocaine in the context of workplace, roadside, and anti-addictive drug

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compliance. Argon plasma treatment improved the meso-pGe efficiency as a SALDI-MS

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substrate and eliminated the need for surface functionalization. The resulting substrate showed a

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precise surface geometry tuning by altering the etching parameters, and an outstanding

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performance for cocaine detection with a limit of detection (LOD) in milliQ water of 1.7 ng/mL

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and in spiked saliva as low as 5.3 ng/mL. The meso-pGe substrate had a demonstrated stability

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over 56 days stored in ambient conditions. This proof-of-principle study demonstrates, that

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meso-pGe can be reproducibly fabricated and applied as an analytical SALDI-MS substrate

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which opens the door for further analytical and forensic high throughput applications.

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1. INTRODUCTION

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Roadside and workplace drug testing has been introduced on a global scale due to the

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increased abuse of illicit compounds. In 2004, Australia introduced roadside drug screening for

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methamphetamine, 3,4-methylenedioxymethamphetamine and tetrahydrocannabinol.1-3 Even

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more recently the United Kingdom has introduced cocaine screening from saliva at roadside

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testing events. Furthermore, cocaine has been reliably detected from urine4 and plasma5 and even

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more recently from hair6 and fingermark sweat.7 However, the accessibility of saliva for rapid

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and non-invasive sampling makes it an attractive biological fluid for detecting illicit drugs.

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Recent studies have shown that cocaine levels in saliva correlate with blood levels and with

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pharmacologic effects following intravenous administration.8 Cocaine is recognized as one of

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the most dangerous illicit drugs with powerful addictive, psychoactive and stimulant properties.

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Statistics confirmed that 7.3% of Australians aged 14 years and over have used cocaine one or

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more times in their lifetime.9 Cocaine and its metabolites have been detected in saliva after oral

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and intravenous administration using immunoassay screening devices with a cut-off limit of 150

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ng/mL.10 However, these tests are presumptive, requiring further confirmatory testing using gas

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chromatography-mass spectrometry (GC-MS) or liquid chromatography mass spectroscopy (LC-

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MS). Furthermore, most commercial immunoassay screening tests for oral fluids can cross react

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with licit drugs, which reduces test specificity and increases the possibility of false positives.11

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Whilst GC-MS and LC-MS are sensitive and accurate for the detection of many illicit drugs,

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sample preparation is time-consuming, requiring extraction and (for GC-MS) derivatization steps

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using costly analytical reagents.12

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Matrix-assisted laser desorption ionization-time of flight-mass spectrometry (MALDI-ToF-

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MS) is a well-established analytical technique used for the analysis of a wide range of large


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molecular weight compounds.13 In 1985, Karas and Hillenkamp adapted laser desorption

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ionization mass spectrometry (LDI-MS) using an aromatic acid as a matrix to absorb the incident

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laser energy and transfer it to the analyte of interest facilitating the desorption/ionization

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process.14-16 In the low molecular weight range, MALDI mass spectra are convoluted due to the

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matrix related fragment and adduct peaks, making the MALDI-MS based detection and

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interpretation of small molecules ( 0.999) was observed demonstrating quantification of cocaine

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using meso-pGe. The LOD for cocaine in water on meso-pGe SALDI substrates was comparable

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to the reported LOD for DIOS substrates (0.86 ng/mL)28 despite the WCA being considerably

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lower in comparison to both DIOS (WCA≈124o) and the commercially available NALDI

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substrates (WCA≈138o). For DIOS and NALDI, a hydrophobic surface is required as it promotes

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the concentration and extraction of hydrophobic drug molecules.24

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compare the S/N observed for cocaine (303 pmol) for meso-pGe substrates with NALDI and

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DIOS.

ANOVA was used to

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Despite the surface hydrophilicity of our meso-pGe SALDI substrate (WCA 62o), the average

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S/N ratio for cocaine (303 pmol) was higher (p< 0.01) compared to both NALDI and DIOS

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substrates (Figure S8, Table S2).

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Figure 6. A) Meso-pGe SALDI mass spectrum of cocaine (MH+ 304, 200 ng/mL) on meso-pGe

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(2000 shots of spectra). The star represents the cocaine protonated peak and the pentagon

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corresponds to a cocaine fragment observed at an ion of m/z 182. B). Linear regression curve for

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cocaine in water for concentrations ranging from 1–200 ng/mL. Error bars correspond to the

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standard deviation for n = 3, (2000 shots of spectra).

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Following the successful determination for the LOD and LOQ of cocaine in water, cocaine in

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spiked saliva at clinically relevant concentrations ranging from 1-200 ng/mL was investigated in

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order to test the performance of meso-pGe SALDI-MS using a real sample matrix (Figure 7).

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Figure 7. A) Meso-pGe SALDI mass spectrum for cocaine (200 ng/mL) spiked in saliva in the

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presence of 50 ng/mL of cocaine–d3. The star indicates the protonated mass for cocaine (MH+

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304), the circle represents cocaine fragment and the pentagon is the deuterated cocaine-d3 (MH+

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307). B). Linear regression curve for cocaine in saliva for concentrations ranging from 1–200

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ng/mL. Error bars correspond to the standard deviation for n = 3, (2000 shots of spectra).


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A representative meso-pGe SALDI mass spectrum for cocaine at 200 ng/mL in the presence of

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cocaine-d3 (50 ng/mL), as the internal standard is presented in Figure 7A. The star represents the

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abundant ion for cocaine, whilst additional peaks at an ion of m/z 182, are characteristic cocaine

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fragment peaks, and the pentagon represents the peak for cocaine-d3 (m/z 307). In principle, the

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simultaneous detection of both cocaine and cocaine-d3 demonstrates that SALDI-MS can be

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used for the detection of multiple small molecules at the same time. A high S/N base peak for

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cocaine was observed, with a LOD and LOQ of 5.3 ng/mL and 12.1 ng/mL, respectively, and

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good linearity (R2=0.995, Figure 7B) which demonstrates the prospect of quantitative detection

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of cocaine in saliva. Indeed, the observed LOD for cocaine in saliva using plasma-treated

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hydrophilic meso-pGe substrates is comparable to the LOD for perfluorinated hydrophobic

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DIOS substrates (3.9 ng/mL).28 Furthermore, the LOD for cocaine in saliva using confirmatory

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meso-pGe SALDI-MS analysis is approximately 30 times lower than the current cut-off limit

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defined for the RapiScan salivary immunoassay, which is used for screening at roadside and

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workplace testing events.40

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4. CONCLUSIONS

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This manuscript details the reproducible fabrication of meso-pGe substrates for the sensitive

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and consistent detection of cocaine using SALDI. HF concentration and current density were

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specifically optimized and compared in terms of pore size to increase S/N for cocaine detection.

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Facile plasma cleaning techniques produced a clean meso-pGe SALDI substrates. Furthermore,

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we have achieved a LOD for cocaine in water and saliva of 1.7 ng/mL and 5.3 ng/mL,

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respectively, and LOQ for cocaine in water of 3.5 ng/mL and in saliva of 12.1 ng/mL without

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any surface chemistry treatments such as silanization and with comparable performance to


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DIOS. This work opens new vistas for further investigations for Ge nanostructures as SALDI-

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MS platform for analytical and forensic applications overcoming the multiple fabrication steps

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required for DIOS substrates. We believe that the sensitivity of this hydrophilic surface may

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engender this technology towards the detection of hydrophilic small molecules that are difficult

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to detect on hydrophobic SALDI surfaces. Indeed, we believe that meso-pGe SALDI substrates

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could be used for high throughput drug detection in the forensic field and beyond. While, in this

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paper, meso-pGe substrates were used for proof-of-principle SALDI applications we believe the

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concise and reproducible fabrication procedures outlined here will allow the further application

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of meso-pGe in its intended fields for biosensing, photovoltaics, and optoelectronics, which will

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be the subject of future experiments.

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5. Associated content Supporting information:

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The effect of varying current density at fixed hydrofluoric acid concentration on mesoporous germanium (meso-

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pGe) average pore size; The effect of altering HF concentration ratio at a fixed current density on average pore size;

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SEM of meso-pGe substrates fabricated using an electrolyte solution of 2:1 HF:EtOH at different current density;

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SALDI MS detection of cocaine on all optimized meso-pGe substrates; contact angle for untreated meso-pGe;

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contact angle measurements for plasma argon treated meso-pGe corresponding to the ageing test measurements;

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XPS survey spectra for meso-pGe pre- and post- treatment; SALDI MS spectra of methadone, methamphetamine,

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oxycodone and metformin on meso-pGe SALDI substrate; comparison of meso-pGe, DIOS and NALDI for cocaine

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detection and the statistical analysis of the corresponding results.

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Auther Information:

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Corresponding Auther

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Email: [email protected]


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Acknowledgements

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This research was conducted and funded by the Australian Research Centre of Excellence in

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Convergent Bio-Nano Science and Technology (Project No. CE140100036) and an Australian

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Research Council Linkage Project (LP110020044).

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Table of Contents (TOC) Graphic

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