Elsevier

Sensors and Actuators B: Chemical

Volume 267, 15 August 2018, Pages 457-466
Sensors and Actuators B: Chemical

Highly sensitive SERS quantification of organophosphorous chemical warfare agents: A major step towards the real time sensing in the gas phase

https://doi.org/10.1016/j.snb.2018.04.058Get rights and content

Highlights

  • Simple, reproducible and cost-effective SERS-based sensor.

  • Real time detection of G-agents surrogate at extremely low concentration in gas phase.

  • SERS substrates based on Au NPs layers with near optimum inter-particle distances.

  • Citrate coating acts as an effective concentrator of target molecules.

Abstract

A surface-enhanced Raman scattering (SERS)-based sensor was developed for the label-free real-time gas phase detection of dimethyl methylphosphonate (DMMP); a surrogate molecule of the G-series nerve agents which are of particular concern due to its extreme toxicity, persistence and previous deployment. The SERS platform was designed using simple elements (Au nano-particles) coated with a citrate layer, and a self-assembly procedure that yields near- optimum distances among the nanoparticles. The citrate coating acts as an effective trap of the target molecules on the immediate vicinity of the Au nanoparticle surface under ambient conditions by reversible hydrogen bonding type interactions. For the first time, we have been able to detect sub-ppm concentrations of DMMP in gas phase (130 parts-per-billion), as might be found on potential emergency scenarios. The high sensitivity, simple preparation and reusability of the SERS platforms developed in this work open up the way for immediate detection of chemical warfare agents in realistic scenarios.

Introduction

The G-series nerve agents, i.e. Tabun (GA), Sarin (GB), Soman (GD), are widely recognized as one of the most toxic group of chemical warfare agents (CWA) due to the presence of organophosphorus esters causing systemic effects predominantly on the central nervous system. The potential use of highly volatile Sarin (Propan-2-yl methylphosphonofluoridate) by terrorist groups [1,2] has raised considerable public concerning the last decades. In 1995, the Tokyo subway attack performed with Sarin killed 12 people, severely injured 50 and caused temporary injuries in nearly 5000 others [3]. Subsequently, Sarin has been used as a weapon in the Irak (2004) and Syria (2013, 2017) wars. Sarin acute exposure guideline level (AEGL-3) is 0.064 ppmV for an exposure of 10 min [4], i.e. the persons expose above this concentration for this period could experience life-threatening health effects or death.

Nowadays, the competing technologies for the rapid detection of chemical agents in gas phase, based on gas chromatography–mass spectroscopy [5,6] and ion mobility spectrometry [7,8], face severe limitations in terms of specificity, portability, cost and simplicity. In general, first responders (FRs) do not have the necessary equipment to perform a rapid and reliable “on site” diagnosis of the scene when dealing with CWAs threats [9]. In addition, the pressure for a fast identification of potential lethal agents in real operational conditions often comes at the expense of the reliability of detection, leading to false alarms that cause chaos and disruption. The review of existing real cases of CWAs attacks clearly indicates that the consequences of non-accidental releases of harmful volatile substances are strongly dependent on the performance of detection systems. It is therefore of high importance to develop methods for the fast and reliable detection of Sarin and similar gases, in such a way that rapid emergency action can be taken.

Surface Enhanced Raman Scattering (SERS) is one of the leading techniques for label-free ultrasensitive vibrational fingerprinting of a variety of molecular compounds [10,11,12]. In the field of explosives and chemical threat detection [13,14], SERS has been identified as key technology thanks to distinctive features such as: ultrahigh sensitivity, detection from a wide variety of matrices and quantification of multiple species in a single measurement, allowing real time detection in the field. Because of its exceptional attributes, a significant impact on point of use homeland security applications is foreseen.

Surface selectivity is key in SERS as strongly enhanced Raman scattering only occurs in very close vicinity (typically less than 10 nm) of the metal when localized surface plasmon nodes are excited [15]. This is even more important for chemical warfare agents (CWAs) that are regarded as poor Raman scatterers [16] with cross-sections in the range of 10−29 cm2 sr−1 molecule−1. Thus, analyte detection at low concentrations is highly challenging unless some effective form of confinement near the surface can be achieved.

Liquids often provide a suitable medium to concentrate the desired molecules in the vicinity of the metal surface. In fact label-free detection in liquid phase of distilled mustard, Sarin and Tabun [[17], [18], [19]] at trace levels on metallic nanostructures has been successfully demonstrated. Particularly outstanding is the detection of femtomol quantities of nerve gases with SERS substrates consisting of flexible superhydrophobic Au-covered Si nanopillars. Similar substrates were applied for ethanol and acetone vapor sensing although in this case the values obtained for limit of detection (LOD) were much higher, i.e 1815 ppmV and 3300 ppmV, respectively [20]. SERS detection of hydrogen cyanide (5 ppmV), was successfully achieved [21] on Au coated Si nano-pillar substrates (400 nm in height, 50 nm wide, 18 pillars mm−2). As could be expected, surface concentration plays a decisive role. The Au film irreversibly traps HCN through the formation of stable [Au(CN)2]-complexes.

The label free SERS detection of target analytes in the gas phase [22] becomes even more challenging for molecules with a low cross section. SERS detection of dimethyl methylphosphonate (DMMP), a G-agents simulant, was demonstrated two decades ago [23] on roughened silver oxide substrates but detection was carried out upon 40 min exposure time and at a concentration of 1000 ppmV. On the other hand, detection of 5000 ppmV of 2-chloroethyl ethyl sulphide (CEES), simulant for HD mustard, has been reported on bare AgFON substrates upon 24 h exposure time [24]. These authors were able to decrease the time scale for unambiguous detection to 15 min by means of functionalization with SAM layers of decanethiol. The sensitivity improvement was attributed to the more favourable orientation of the CEES molecules on functionalised substrates. A considerable advance towards real-time gas detection was demonstrated with 8 ppmV benzenethiol (CWA precursor) on SERS-active AgFON substrates [25] (200 nm Ag film onto 600 nm Silica nanospheres), thanks to the irreversible chemisorption of benzenethiol through a strong S-Ag bond.

SERS detection at trace concentration levels in gas phase is hampered by the fact that only a few molecules of interest are localized at the enhancing surface. Many strategies address this problem including the above mentioned establishment of chemical bonds with the surface. Sometimes, the sample surface can be cooled to cause condensation on the SERS substrate [26], although this approach is not feasible for field use where low power consumption and fast detection at ambient conditions are required. Alternatively, the SERS can be functionalized for trapping of target analyte. Typical strategies rely on the use of either capture layers to provide high-affinity binding sites or partition layers with low affinity and rapidly reversible binding sites. Thus, the combination of metal-organic framework (ZIF-8) with SERS active structures has been recently attempted [27,28] for room temperature detection of VOCs. The experimental results reveal evidence for the favourable but reversible interactions between the aromatic compounds and the MOF surface within the sensing volume of SERS, leading to 540 ppmV as LOD for benzene. Similarly, the continuous monitoring of airborne chemicals has been investigated on metallic substrates coated with different polymeric layers [[29], [30], [31], [32]]. In particular, the presence of hydrophobic thiols promoted the interaction with aromatic compounds resulting in enhanced SERS signals. Similarly, PDMS layers on the metallic surface captured vapor molecules from flowing air, placing them in close proximity to the plasmonic hot spots and thus, the thickness of the PDMS film became a critical factor on detection performance. The sorptive material can also be used as nanoparticle support. Thus, Au nanoparticles formed on reduced graphene oxide were successfully tested for the detection of VOCs biomarkers in exhaled breath [33]. The graphene oxide provided both fluorescence quenching and surface area for sorption of analyte molecules and nanoparticle dispersion [34]. The above overview shows that rapid detection of target molecules at low concentrations in air is still highly challenging, especially if it is to be carried out on recyclable cost-effective SERS substrates [35].

In the present work, highly sensitive SERS quantification of DMMP has been demonstrated on citrate-capped Au nanoparticle monolayers (Scheme 1) at extremely low concentration, e.g. part-per billion, in gas phase. To obtain the required surface density and distribution of Au nanoparticles a layer by layer adsorption process was optimized using PDDA as cationic polyelectrolyte. The citrate layer on the Au nanoparticles performed as an effective molecular trap by reversible DMMP adsorption on the vicinity of plasmonic Au NPs allowing continuous operation of the SERS sensing platform.

Section snippets

Synthesis of Au NPs

The citrate capped Au nanoparticles were synthesized with a reaction yield of 97% via a modified version of the Turkevich-Frens method [[35], [36]]. 50 mL of aqueous solution (1.1 mM) of HAuCl4 (50% Au basis) was heated to 70 °C under stirring, and then 5 mL of preheated sodium citrate solution (3.8 mM) was added. The solution was kept at 70 °C until a red-wine colour appeared, circa 10 min. Then, the liquid was allowed to cool to room temperature. The synthesis experiments have been performed

Synthesis and deposition of Au@citrate NPs

The citrate capped Au nanoparticles were synthesized with a reaction yield of 97% via modified version of the Turkevich-Frens method [[35], [36]]. The morphology of the Au@citrate nanoparticles was examined by TEM (Fig. 1A and B) and the diameter of at least 100 particles from two different batches was determined from TEM images. The size distribution has an average size of 22 ± 9 nm (Fig. 1C). The elongation of the particles was typically around 1.22; i.e. gold particles can be described as

Conclusions

In summary, a simple and affordable label-free SERS based sensor has been devised for the highly sensitive quantification of dimethyl methylphosphonate in the gas phase via self-assembled high-density Au@citrate monolayers on SiO2/Si substrates. The citrate coating was instrumental in trapping the desired molecule near the surface of the Au nanoparticles by means of reversible hydrogen bonding interactions. On the other hand, the facile preparation procedure developed in this work yielded

Conflicts of interest

Authors declare that there are no conflicts of interest.

Acknowledgements

Financial support from financial support from MICINN (CTQ2013-49068-C2-1-R;CTQ2016-79419-R) and CUD (UZCUD2016-TEC-07) is gratefully acknowledged. CIBER-BBN is an initiative funded by the VI National R&D&i Plan2008-2011 financed by the Instituto de Salud Carlos III with assistance from the European Regional Development Fund.

Marta Lafuente received her M.Sc. (2015) degree in Nanostructured Materials for Nanotechnology Applications from the University of Zaragoza (Spain). She is currently a pre-doctoral student at the Institute of Nanoscience of Aragon (INA). Her thesis is focused on Detection of Chemical Warfare Agents in vapor phase using Surface Enhacement Raman Spectroscopy (SERS).

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    Marta Lafuente received her M.Sc. (2015) degree in Nanostructured Materials for Nanotechnology Applications from the University of Zaragoza (Spain). She is currently a pre-doctoral student at the Institute of Nanoscience of Aragon (INA). Her thesis is focused on Detection of Chemical Warfare Agents in vapor phase using Surface Enhacement Raman Spectroscopy (SERS).

    Ismael Pellejero received his Ph.D. (2012) degree in chemical engineering from the University of Zaragoza (Spain). He is currently Junior Researcher in the Institute for Advanced Materials INAMAT at the Public University of Navarre (Spain). His research interests include photocatalysis and microsystems fabrication, mainly chemical sensors and microreactors, incorporating nanoporous materials as functional coatings.

    Victor Sebastián is Associate Professor at the University of Zaragoza (Spain). He received PhD (Summa Cum Laude) degree in Chemical Engineering from University of Zaragoza (Spain) in 2008. During his PhD studies, he was invited researcher in University of Aveiro (Portugal-2004), SINTEF-Oslo (Norway-2005) and IMM(Germany-2005). He was a postdoctoral-research fellow working at IEM-CNRS (France-2008). He was awarded as a Fulbright Fellow to start his research in microfluidics and nanomaterials synthesis at MIT (USA-2009–2011). His research interests include the synthesis of a wide variety of nanostructures using microfluidics and their application for energy conversion, catalysis and biomedical uses.

    Miguel Urbiztondo received his Ph.D. (2008) degree in Chemical Engineering from the University of Zaragoza (Spain). He is Associate Professor at the Centro Universitario de la Defensa Zaragoza (Spain) and member of the Institute of Nanoscience of Aragon since its creation in 2003. His research interests are focused in the development of microdevices based on nanostructured materials for sorption, separation, reaction and sensing applications and molecular dynamics simulation applied to microwave driven heterogeneous catalysis.

    Reyes Mallada received his PhD in Chemistry in 1999 from the University of Zaragoza, (Spain) and has done postdoctoral stays at the University of Southern California (USA) and University of Twente (The Netherlands). She is Associate Professor in the Chemical and Environmental Engineering Department at the University of Zaragoza since 2007 and member of the Institute of Nanoscience of Aragon since its creation in 2003. She is currently working on catalyst activation by different energy forms including microwaves and light, catalytic microreactors, nanostructured membranes and chemical sensors.

    María Pilar Pina is Associate Professor with tenure (2007) in the Department of Chemical and Environmental Engineering at the University of Zaragoza (Spain) and member of the Institute of Nanoscience of Aragon (INA) since its creation in 2003. Her current research lines pivoted on microsystems in i) chemical engineering for sorption, reaction and separation applications based on nanostructured and porous materials, and ii) new strategies for sensing and catalysis based on plasmonic nanomaterials in the fields of homeland security, air quality control and healthcare applications. She has participated in 39 research projects, in 17 of them as principal researcher, at national and international level. From the results of these projects she has published 64 peer-reviewed manuscripts, filed 4 patents and delivered over 150 presentations at scientific meetings. Her current Hirsch index number is h = 17 (ISI-WOK) or 20 (Google Scholar).

    Jesus Santamaría, is the Vice-Director of the Institute of Nanoscience of Aragon (INA), Samca Professor of Nanotechnology and Editor of the Materials Synthesis section of the Chemical Engineering Journal. His group Nanostructured Films and Particles (NFP) is among the leading European groups in advanced Reactor Engineering and has made pioneering contributions in the fields of membrane reactors, zeolite membranes, microreactors, microwave and light-assisted catalytic reactors and process safety, including nano-safety aspects. Prof. Santamaria has been acknowledged by a number of awards and other forms of recognition. Also, he is a frequently invited speaker in international conferences (Plenary or Keynote speaker in 68 scientific meetings). He has taken part in 91 research projects, including two ERC Advanced Grants (HECTOR, 2011-16 and CADENCE, 2017-22). From the results of these projects he has published 302 peer-reviewed manuscripts, filed 24 patents and delivered over 400 presentations at scientific meetings. His current Hirsch index number is h = 45 (ISI-WOK) or 54 (Google Scholar).

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