# Copper-decorated porous silicon for the bimodal detection of DMMP, a sarin simulant, by FT-IR spectroscopy and matrix-free laser desorption ionization mass spectrometry

Warda Raiah, Mohammed Guendouz, Parastesh Piratesh, Vincent Thomy,

Joël Charrier, Yannick Coffinier

► To cite this version:

Warda Raiah, Mohammed Guendouz, Parastesh Piratesh, Vincent Thomy, Joël Charrier, et al. Copper-decorated porous silicon for the bimodal detection of DMMP, a sarin simulant, by FT-IR spectroscopy and matrix-free laser desorption ionization mass spectrometry. Microchemical Journal, 2025, 217, 10.1016/j.microc.2025.115034. hal-05241214

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# Copper-decorated porous silicon for the bimodal detection of DMMP, a sarin simulant, by FT-IR spectroscopy and matrix-free laser desorption ionization mass spectrometry

Warda Raiah<sup>a,b</sup>, Mohammed Guendouz<sup>b</sup>, Parastesh Piratesh<sup>b</sup>, Vincent Thomy<sup>a</sup>, Joël Charrier<sup>b</sup>, Yannick Coffinier<sup>a,*</sup>

## ARTICLE INFO

### Keywords:

Porous silicon

DMMP

FT-IR spectroscopy

Matrix-free LDI mass spectrometry

Copper particles

## ABSTRACT

The release of organophosphates into the natural environment may directly and indirectly affect human health and cause severe damage to environmental ecosystems, thus raising global concern. In addition, some of these organophosphorus compounds (OPs) have been weaponized. Among them, Sarin, which is an OP-based nerve agent, has already been used as massive chemical weapons emphasising the need for their detection. In this study, we propose the development of a new nanomaterial based on porous silicon (PSi) decorated with copper particles for the detection of dimethyl methylphosphonate (DMMP), a sarin simulant. We took advantage of the large surface area of PSi and the specificity towards DMMP brought by copper particle decoration. Two types of PSi porosities (60 % and 80 %), were used while experimental copper particle deposition method was evaluated to ensure a uniform distribution within porous texturation. Then, and for the first time, the presence of DMMP was detected bimodally using FT-IR and mass spectrometry from the same PSi-Cu surface. We demonstrated enhanced specificity for DMMP detection using porous silicon decorated with copper particles (PSi-Cu) compared to unmodified PSi. Several characteristic IR absorption bands of DMMP were clearly observed, including peaks at 1319 and 925 cm⁻¹ corresponding to the symmetric and asymmetric deformation modes of the P–CH₃ group, the P–O–C stretching mode at 830 cm⁻¹, and the P=O stretching vibration at 1246 cm⁻¹. In addition, the PSi-Cu surface enabled matrix-free surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS), revealing a strong interaction between DMMP and copper. The dominant mass spectral peak at *m/z* = 186, assigned to the [M + Cu]⁺ adduct, confirms a Lewis acid-type interaction between DMMP and the Cu-decorated surface. This study aimed to demonstrate, for the first time, the feasibility of combining FT-IR spectroscopy and matrix-free SALDI-MS for DMMP detection on a single, untreated PSi-Cu platform. Although the sensitivity, response time, and recovery time of the system are not yet state-of-the-art, our results highlight the potential of this dual-mode analytical approach for analyzing complex samples on same surface making it an invaluable tool in fields such as chemistry, pharmaceuticals, forensics, and environmental science.

## 1. Introduction

Human activities generate a very large quantity of chemical pollutants (dyes, synthetic intermediates, phytosanitary products, persistent organic pollutants (POPs, e. g. PFOA), drugs, etc.). Among these compounds, pesticides are one of the only classes of xenobiotics that are ‘voluntarily’ discharged into the natural environment for agricultural production purposes [1,2]. Considering the past and current uses,

pesticides (e.g. glyphosate) are represented by more than a thousand active substances, very heterogeneous both in terms of physicochemical characteristics, fate in the environment and their mechanism of toxic action. Most of these pesticides are organophosphorus compounds. The widespread use of such compounds for several decades has led to their dissemination in all environments of the biosphere (water, soil and atmosphere), with, for some, a long-term persistence. Their chronic toxic effects on the living beings of the terrestrial and aquatic biomass are the

<footnote><sup>a</sup> Univ. Lille, IEMN, UMR CNRS 8520, Avenue Poincaré, BP 60069 59652, Villeneuve d'Ascq, France</footnote>

<footnote><sup>b</sup> Univ Rennes 1, CNRS, Institut Foton – UMR, 6082, F-22305, Lannion, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: yannick.coffinier@univ-lille.fr (Y. Coffinier).</footnote>
result of their accumulation in seeds, plants and fruits but also in the soil, with potential contamination of drinkable water. In humans, pesticides enter the body through three routes: the skin, the digestive (or oral) route and the respiratory route. In addition to direct health effects such as headaches, irritation and abdominal pain, epidemiological studies have shown the involvement of pesticides in several serious pathologies in people occupationally exposed to these substances, in particular: cancer (prostate cancer, breast cancer, certain haematopoietic cancers such as non-Hodgkin's lymphoma and multiple myeloma), neurological diseases (Parkinson's disease) and reproductive disorders but also during critical periods of fetal development in utero [3-5]. In addition to their agricultural use, some of these OPs can be or have been already used as chemical weapons as nerve agents (e.g. Sarin). Once weaponized, they become large-scale weapons of mass destruction [6]. Even though their use was prohibited by the Geneva Protocol (1925), recent events in Syria (1430 victims) and the 1995 attack by the Aum sect in the Tokyo subway (12 dead, 5500 wounded) have shown that these neurotoxins, especially sarin, are still a current threat for populations (bioterrorism) but also for soldiers in the field. Currently, one of the methods of choice for the detection of OPs is the measurement of acetylcholinesterase (AChE) activity or rather its inhibition in the presence of neurotoxins (impeding neuromuscular transmission) [7]. Indeed, the presence of pesticides inhibits the conversion of acetylcholine into choline. The intensity of AChE inhibition is proportional to the concentration of OPs so this could be exploited as a detection principle. Most existing sensors measure AChE activity using amperometric, potentiometric, optical (SPR, photoluminescence) and fluorimetric (QDots)/colorimetric methods with sensitivities down to the nanomolar range. Most of these methods use the redox properties of hydrogen peroxide ($H_2O_2$) (product resulting from the conversion of choline to betaine in the presence of $O_2$ and $H_2O$) by choline oxidase (ChO) [8,9]. Nevertheless, one of the main drawbacks is that this approach does not allow direct detection of the presence of neurotoxics and that there are often interfering molecules in the sample to test, i. e. compounds reducing or oxidizing at the same potential as hydrogen peroxide. This can lead to false responses. Moreover, the use of enzymes also poses several problems such as the overall cost of the sensor itself and the need to provide stable anchoring of the enzyme so that it can be either physisorbed, trapped within a hydrogel (to maintain good operational conditions) or covalently bound, thus ensuring good catalytic site orientation and so reliable enzymatic activity and sensor performance. However, other alternatives have been developed like mechanical-based sensors with the use of a quartz microbalance or cantilever measuring the oscillation frequency shift, as well as those based on the change of conductance of electrical devices (carbon nanotubes, organic field effect transistor) when exposed to different OP simulants [10-15]. Combined methods have also been developed such as localized surface plasmon resonance (LSPR) coupled with electrochemical impedance (EIS) measurements [16]. Nevertheless, mass spectrometry (MS) remains the gold standard method with high accuracy but still suffering of tedious sample preparation and the need of heavy equipment and fluid lines (gases, liquids) making its miniaturization difficult and thus limiting its portability [17-19].

Like these agents, DMMP contains a phosphorus-oxygen ($P=O$) functional group, which is critical for its interaction with detection surfaces and sensors. Due to its similar physicochemical properties but lower toxicity, DMMP is thus commonly employed in laboratory settings to safely study detection strategies for chemical warfare agents. To enhance DMMP detection, sensor surfaces and materials are most of the time modified with organic (self-assembly monolayer (SAM)) and inorganic materials (metal oxides). For instance, it has been shown that $Cu^{2+}$-chelated ions can form a strong coordination bond with $P=O$ enable rapid, sensitive and reversible and reproducible detection of simulants like DMMP [20]. Additionally, hydrogen-bonding agents like HFIPP enhance specificity, while metal oxide semiconductors (e.g., ZnO, $SnO_2$, $WO_3$, $MoS_2$, $TiO_2$, CuO) offer stability and ease of fabrication, further improving sensor performance [21-26]. So, here, we present a

new nanomaterial based on porous silicon decorated with copper particles for the detection of DMMP. To do so, we took advantage of the huge surface area of PSi together with the specificity towards DMMP introduced by copper oxide particles. The presence of DMMP was detected for the first time in dual mode from the same PSi-Cu surface, first by FT-IR spectroscopy and then confirmed by matrix-free laser desorption/ionization mass spectrometry. In the present study, rather than seeking to maximize performance metrics such as limit of detection or sensitivity, the aim of our work was to demonstrate for the first time the feasibility of combining FT-IR spectroscopy and matrix-free SALDIMS on a single copper-decorated porous silicon platform without any additional surface treatment.

## 2. Materials and methods

### 2.1. Chemicals

Ethylenediaminetetraacetic acid (EDTA), bis-(3-sulphopropyl) disulphide, disodium salt (SPS), polyethylene glycol (PEG) (4000 g/mol), DMMP, $CuCl_2$ and $CuSO_4 \cdot 5H_2O$ were all purchased from Sigma-Aldrich and used as received. Hydrofluoric acid (HF; 50 %) was purchased from Honeywell and acetone, dichloromethane ($CH_2Cl_2$), isopropyl alcohol, methanol, ethanol (EtOH) and hydrogen peroxide (30 %, w/w) were purchased from Carlo Erba. Ultrapure water (Milli-Q, 18 MΩ cm$^{-1}$) was used in the preparation of the solutions and in all rinsing steps when needed.

### 2.2. Porous silicon fabrication

PSi layers were obtained by electrochemical anodization of p-type doped (100) oriented silicon wafer (5.10–3 Ohm.cm) at room temperature. The electrolyte was composed of HF (50 %) in ethanol and deionized water with the following ratio 2:2:1. The refractive indices and porosities of PSi single layers were studied by reflectometry in the near- and mid-infrared spectral ranges. From reflectance spectra, the refractive index for each of the fabricated single PSi layers was deduced by fitting experimental spectral fringes using the effective medium theory (Bruggeman model). The thickness of each porous layer was controlled by the anodization etching time. Current densities of 20 and 90 mA/cm$^2$ were used for the low porosity (60 %) and high porosity (80 %), respectively. To open the pores, freshly prepared porous silicon was etched to remove a thin layer of low porosity by $CHF_3$ RIE - ICP plasma.

### 2.3. Copper particle decoration

PSi surfaces were dipped in a solution of copper salt ($CuCl_2$ and $CuSO_4 \cdot 5H_2O$; 0.035 M) and HF (1.45 M) for 60 s at room temperature. Then, surfaces were rinsed with deionized water to stop the deposition of Cu particles and dried under a gentle stream of nitrogen. The time of deposition and copper salt concentrations were adjusted first to ensure a homogenous deposition without affecting the overall morphology of PSi. Indeed, since we are using a mixture of copper salt and HF, a metal-assisted chemical etching process was also concomitantly occurring during copper particle, thus opening the voids within the PSi texturation and modifying the overall surface morphology [27].

### 2.4. Scanning electron microscopy (SEM)

SEM images were obtained using a Zeiss ULTRA 55 electron microscope equipped with a thermal field emission emitter with three different detectors: an EsB detector with s filter grid, a high-efficiency In-lens SE detector and an Everhart-Thornley Secondary Electron Detector.

### 2.5. Measurement of water contact angle (WCA)

WCA values were measured using deionized water and a DIGIDROP
<figure><img src="image_3.png" /><figcaption>Fig. 1. Optical and SEM images of PSi with 60 % (A, C) and 80 % porosity (B, D).</figcaption></figure>

goniometer system (GBX, France), with accuracy of ±2°. All measurements were made in the ambient atmosphere at room temperature on three different locations over each surface.

## 2.6. Fourier-transform infrared (FT-IR) spectroscopy

Specular reflection FT-IR spectra were recorded using a Perkin-Elmer Spectrum 2000 single-beam spectrophotometer equipped with a tungsten-halogen lamp and a liquid nitrogen-cooled MCT detector. All measurements were made after purging the sample chamber for 30 min with dry N₂. Spectra were recorded at 4 cm⁻¹ resolution and averaged over 200 scans. Background spectra were obtained using a flat untreated PSi surface.

## 2.7. X-ray photoelectron spectroscopy (XPS)

XPS measurements were performed with an ESCALAB 220 XL spectrometer from Vacuum Generators. A monochromatic Al K X-ray source (1486.6 eV) was operated in constant analyser energy (CAE) mode (CAE = 100 eV for survey spectra and CAE = 40 eV for high resolution spectra) using the electromagnetic lens mode. The angle between the incident X-rays and the analyser is 58°. The detection angle of the photoelectrons is 90° relative to the sample surface and the C1s at 285 eV was used for binding energy correction.

## 2.8. X-ray diffraction (XRD)

XRD patterns were recorded using a high-resolution Rigaku X-ray diffractometer (model: ULTIMA IV, Rigaku, Japan) in the angular variation of 10–70° with a Cu Kα X-ray radiation (λ = 0.15406 nm).

## 2.9. MS measurements

After interaction of PSi surfaces with DMMP in the reaction chamber, each PSi surface was fixed on an MTP TLC adapter (Bruker) with carbon conductive double-sided adhesive tape. Then the plate was directly introduced into the mass spectrometer ion source. Commercial peptide mixtures were used for the mass spectrometer calibration. The

calibration curve and relative standard deviation calculations were obtained from inter-spot (I ≥ 6), intra-spot (I ≥ 3) and surface-to-surface (I ≥ 3) mean values. MALDI mass spectra were recorded on a Bruker Autoflex Speed instrument (Bruker Daltonics, Wissembourg, France) equipped with a time of flight (ToF) analyser and a LIFT module. A pulsed Smartbeam II 2 kHz laser at a wavelength of 355 nm (~3.49 eV) was operated at a frequency of 1000 Hz (MS data) with a delayed extraction time of 350 ns. The source was operated in the positive mode. An acceleration voltage of 25.0 kV (IS1) was applied for a final acceleration of 21.95 kV (IS2). Random laser irradiation was performed according to the partial sample mode with 1000–5000 shots/point. Ions were detected over different mass ranges according to the samples analysed. Data were acquired with FlexControl software and processed with FlexAnalysis software. The mass range used was below 4000 Da and reflectron mode was used. The signal to noise ratio (S/N) was set to be a minimum of 3, and all the laser percentages represent the absolute value of the laser, not the relative laser power presented in the GUI (graphical user interface) from the FlexControl 3.4 software.

## 2.10. Safety considerations

The mixture of H₂SO₄ and H₂O₂ (piranha solution) is a strong oxidant. It reacts violently with organic materials. It can cause severe skin burns. It must be handled with extreme care in a well-ventilated fume hood while wearing appropriate chemical safety protection.

# 3. Results and discussion

PSi layers were manufactured via electrochemical anodization of p-type doped (100)-oriented silicon wafer (4–6 Ω.cm) at room temperature by using an electrolyte composed of HF, deionized water and ethanol at a ratio of 2:1:2. Electrochemical anodization offers the advantage of controlling the direction of the pores formed in the bulk silicon, through the choice of the type and level of doping of the starting silicon. By varying the anodization time and the current density, various PSi morphologies were obtained. Measuring their refractive indices by reflectometry in the near- and mid-infrared spectral ranges gave values of 1.9 and 1.4 µm @1550 nm that correspond to PSi with 60 % and 80 %
<figure><img src="image_4.png" /><figcaption>Fig. 2. XPS spectra of PSi-Cu surface.</figcaption></figure>

porosities, respectively [28]. In Fig. 1 A and B, optical images show the good uniformity of the obtained porous layer whereas the SEM images in Fig. 1C and D display the surface morphology of both 60 % and 80 % PSi used in this study. The obtained structures are mainly columnar, corresponding to the mesoporous silicon type. At 60 % porosity, the average pore size is 15 ± 10 nm whereas at 80 % it is 60 ± 20 nm.

In order to specifically capture and enrichment ability of OPs, organic (SAM) and inorganic approaches have been already developed. For instance, SAM with chelated Cu²⁺ ions have been used to form strong coordination bonds with P=O from OPs to help its detection [20]. Recent studies have presented the deposition of 1D CuO nanostructures onto cantilevers to detect OPs agents, thus combining the high sensitivity given by microcantilevers with enhancement of the surface area, thanks to the presence of 1D nanostructures together with the specific interaction between CuO and phosphate groups [15,29]. Here, in this regard, we have chosen to investigate the detection of DMMP in the gaseous state, using Cu nanoparticle-decorated PSi, denoted PSi-Cu, taking advantage of the huge surface area of PSi together with the ability to form coordination bonds between Cu particles and OPs. PSi-Cu surfaces were formed via electroless method. To do so, freshly prepared PSi surfaces were dipped in an aqueous solution containing CuCl₂ or CuSO₄·5H₂O (0.035 M) and HF (1.45 M) for 60 s at room temperature. Then, surfaces were rinsed with deionized water to stop the deposition of Cu particles and dried under a gentle stream of nitrogen as already described by Nguyen et al. [30]. The electroless process is a very

straightforward, fast, low-cost and highly reproducible process leading to a relatively good coverage of silicon nanostructures by copper particles (Fig. S1). We can easily distinguish changes in the overall surface morphology, with numerous particles located on top of the nanostructures. Indeed, SEM images show that Cu nanoparticles are well formed on PSi surfaces following reduction of the Cu²⁺ ions contained in the ionic solution into Cu metal deposited particles. We measured the depth of particle deposition to be ~360 nm (Fig. S1, inset). The presence of copper particles was also confirmed by EDX analysis with a characteristic peak centred at 0.95 keV (Fig. S2 in supporting information). The presence of oxygen (O) is explained by oxidation of the deposited copper particles under ambient atmosphere and from the native silicon oxide. The peak assigned to Si comes from PSi. By comparing Cu particles deposition from another copper salt (e. g. CuSO₄·5H₂O), we observed higher Cu signal intensity of PSi-Cu immersed in CuCl₂ compared to that obtained by using CuSO₄ as copper salt (Fig. S2 and inset). As a result, CuCl₂ as the copper salt was used preferentially to achieve better Cu particle deposition on PSi layers. In addition to EDX spectra, the presence of a Cu₂O/CuO layer around the copper particles was confirmed by XPS and XRD analysis (Fig. 2 and S3). Indeed, copper particles get rapidly oxidized and form oxidized Cu particles composed by a crystalline core (XRD) with overlayers of Cu₂O/CuO. The CuO layer around the Cu particles will ensure specific interactions with the DMMP compounds since it contains Cu²⁺ [31]. In Fig. 2 are displayed the XPS survey and high-resolution spectra of O₁s
<figure><img src="image_5.png" /><figcaption>Fig. 3. EDX spectra of PSi-Cu surfaces with 60 % (A) and 80 % porosity (B) for different solvent compositions.</figcaption></figure>

and Cu<sub>2p</sub> elements that clearly show the presence of Cu and confirming the presence of CuO layer as it was already observed in our previous study [31].

For more detail, in the **table S1** in supporting information are listed the elemental composition of PSi-Cu surface that is mainly composed of O<sub>1s</sub> (57.1 %), Si<sub>2p</sub> (24.4 %), Cu (4.8 %), C<sub>1s</sub> (12.8 %) and trace of chlorine Cl<sub>2p</sub> (0.9 %). The static WCA measurements where PSi-Cu displayed hydrophilic behaviour with WCA values ~19–22° which is a bit lower than for the PSi surface (WCA 25°) (**Table S2** in supporting information). It has to be noticed that while aiming to improve Cu particle deposition, we tried to add some chemical additives to the aqueous solution to try improving Cu particle deposition within PSi texturation. Indeed, it is well known that appropriate additives such as EDTA, SPS, PEG, etc. used as a chelating agent, enhancer of the copper plating rate and stabilizer, respectively, can strongly improve the uniformity of copper particle deposition [32]. However, after examination on the influence of additives by adding chemical compounds such as PEG (~4000 g/mol) and SPS at various concentrations no significant improvement of Cu particle distribution in terms of uniformity and

spatial distribution was demonstrated under our experimental conditions.

Nevertheless, to continuously improve the efficiency of our Cu particle deposition, we also assessed the solvent composition by adding methanol to aqueous copper salt solution in order to play with the surface tension and make the solution infiltration easier within PSi texturation. To do so, we used different methanol/H<sub>2</sub>O ratios. Adding a higher amount of methanol led to higher Cu particle deposition as shown in EDX spectra for both porosities (Fig. 3). The best particle deposition was obtained for a copper salt solution made in pure methanol (100 %) for both porosities. Indeed, since methanol has a much lower surface tension (22.5 mN/m) than water (72 mN/m), it has good wetting properties. It seems therefore that methanol was facilitating wetting of the PSi layer and then better promoting Cu particle deposition within the PSi layer.

In Fig. S4 are displayed SEM images of the surface morphology of PSi-Cu surfaces with 60 % and 80 % porosity, respectively, obtained under the same electroless deposition. We also notice that the Cu nanoparticles tend to agglomerate to form Cu films. Moreover, this

<figure><img src="image_6.png" /><figcaption>Fig. 4. FT-IR spectra of DMMP after interaction with PSi and PSi-Cu NP (80 %). Blue curves correspond to vaporization of DMMP @70 °C with interaction time of 3 h under atmospheric pressure. Red curve corresponds to interaction time of 3 h under atmospheric pressure without vaporization of DMMP @70 °C. Pink curves correspond to vaporization of DMMP @70 °C with interaction time of 24 h under atmospheric pressure. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
deposited Cu film is denser on the PSi surface with 60 % porosity which implies total closure of the pores on the surface due to their small size. In contrast, the pores of 80 % porosity PSi are reduced but still remain open after Cu particle deposition.

We also tried to use isopropanol (IPA) instead of methanol, because it has a similar surface tension (23 mN/m) but lower vapor pressure. Indeed, methanol evaporates more quickly than IPA and can lead to drastic modification of the local concentration of copper salt and then provoke less uniform Cu particle deposition. However, the Cu particle deposition was still better with pure methanol and so no significant effect over the deposition with IPA was observed.

Once we found the best conditions for the fabrication of the PSi-Cu surface, we attempted to detect DMMP, a commonly used simulant of Sarin because of its very similar chemical structure. After optimization of the Cu nanoparticle deposition conditions, 80 % porosity PSi-Cu surfaces were exploited for DMMP detection in order to use opened porosity after copper particle decoration. For the detection of DMMP in the gaseous state, we performed several tests to find the optimal interaction conditions. The main conditions that we have considered in this study are interaction time, static, dynamic and under vacuum modes, and temperature (DMMP vaporization at room temperature or 70 °C). Heating DMMP at 70 °C is expected to favour its vaporization. Then, PSi-Cu surfaces with 80 % porosity were tested under the different conditions. The set-up used is schematically described in Fig. S5. 100 µL of pure DMMP was used and left to evaporate either at atmospheric pressure, by heating (70 °C) or by using vacuum pumping for various times from 3 to 24 h. After each interaction time (under static or dynamic mode), FT-IR measurements were performed. As a control surface, we used PSi with 80 % porosity. A phosphoryl stretching band (P=O) from DMMP in the gas phase presents a characteristic peak at 1275 cm⁻¹ whereas in the liquid or solid phase this P=O band position is shifted to 1246 cm⁻¹ which corresponds to typical DMMP adsorbed on metal oxide, i. e. interaction of P=O with metal Lewis acid or hydroxyl groups from the surface [33]. For all the different tests that we conducted, the four characteristic peaks at 1319 and 925 cm⁻¹ were present due to the symmetric and anti-symmetric deformation modes of P-CH₃, respectively, as well as the P-O-C mode at 830 cm⁻¹ and the P=O band at 1246 cm⁻¹ (Fig. 4, right). The P=O stretching centred at 1266 cm⁻¹ in addition to the one at 1246 cm⁻¹ for the experiment performed under vacuum is consistent with a contribution from a second type of interaction mechanism of P=O with the surface [33]. Indeed, according to the literature, the P=O band is known to be sensitive to the surrounding medium and mode of interaction. In the control experiment, none of the characteristic peaks were observed, thus highlighting the good

specificity of PSi-Cu for DMMP. Nevertheless, for a longer time of interaction under vacuum conditions, a peak at 1266 cm⁻¹ was observed (Fig. 4, pink curve) surely due to the P=O band. From all these results, we found that the best conditions for interaction between DMMP and PSi-Cu were 3 h at room temperature and atmospheric pressure with preliminarily vaporization of DMMP at 70 °C (Fig. 4, right blue curve). We have recently shown that PSi coated with TiO₂ layer was also able to detect low amount of DMMP with LoD value of 4 ppm *via* FT-IR spectroscopy [34]. We have shown that the detection performance of our TiO₂-PSi nanomaterials is in line with the immediately dangerous for life or health limit (IDLH) for some pesticides, but still underperforms for the detection of warfare agents. For instance, for malathion and parathion, two commonly used organophosphorus pesticides, the IDLH values are 18.52 ppm and 0.84 ppm, respectively. However, recent studies have reported highly sensitive DMMP sensors with remarkable detection limits and fast responses characteristic by using rGO-Nitrogen enriched carbon nanodots (NCD) based materials containing copper ions *via* electrical resistance modulation [35]. Such materials demonstrated fast (<80 s)) and sensitive (34 ppb) detection of DMMP that could be used for the detection of Sarin, Soman and Tabun for which the IDLH limits are much lower, at approximately 0.03 ppm, 0.008 ppm and 0.03 ppm respectively [36]. However, in our study, the approach is quite different since we are not seeking to outperform already existing methods displaying very low LoD and fast time response and recovery time. Instead, we are proposing to combine 2 analytical techniques namely FT-IR spectroscopy and mass spectrometry in dual mode by using our PSi-Cu. Indeed, while electrical and spectroscopic techniques offer rapid and label-free detection of volatile organophosphonates such as DMMP, their inherent lack of molecular specificity often limits their standalone use. Resistance-based sensors respond to changes in charge carrier mobility or surface potential upon analyte adsorption, but these signals are typically non-specific and sensitive to environmental factors such as humidity or temperature. Similarly, FT-IR or Raman spectroscopies provide vibrational fingerprints of functional groups (e. g., P=O, P-O-C), yet these features often overlap with those of structurally related compounds (their metabolite for instance) or background matrix components. In contrast, LDI-MS enables the detection of molecular and fragment ions that are unique to DMMP, offering high selectivity and unambiguous identification even in the presence of interferent species. Indeed, mass spectrometry can unambiguously distinguish between various CWA simulants bearing a phosphonate moiety such as DMMP, DEEP, DEPA, and PMP even in complex samples, thanks to their distinct *m/z* signatures [37]. Therefore, the integration of LDI-MS into the sensing process is critical to overcome the ambiguities of some signals,

<figure><img src="image_7.png" /><figcaption>Fig. 5. Mass spectra of DMMP obtained from PSi-Cu surface at different laser fluences, A (70 %) and B (80 %).</figcaption></figure>
and to provide a robust and chemically resolved readout of DMMP adsorption events.

Thus, combining FT-IR spectroscopy with mass spectrometry could offer a powerful analytical approach for the comprehensive characterization of compounds. This synergistic combination provides complementary information about the chemical composition, structure, and functional groups present in a sample. We have demonstrated recently that FT-IR spectroscopy can provide valuable insights into the molecular vibrations of functional groups. Indeed, this technique was shown to be particularly sensitive to the presence of bonds such as P=O, P-CH₃, P-O-C allowing for the identification of organic compounds such as DMMP [34]. On the other hand, mass spectrometry can provide precise molecular weight information, thus confirming the presence of the compounds of interest. Indeed, being considered as a gold standard analytical technique because of its high accuracy and sensitivity, detection limits and operational speed, MS technique can accurately identify chemical or biological compounds. To this extent, we propose using SALDI-MS technique to detect DMMP molecules. SALDI is a derivative of MALDI technique where no organic matrix is used to promote the laser assisted desorption and ionization process. Such matrix-free LDI process overcoming the main drawbacks of the classical MALDI process and is particularly well dedicated to the detection of small compounds. The LDI process is thermally driven and so upon laser irradiation (λ = 355 nm), heat confinement within the porous silicon nanostructures facilitates efficient energy transfer to the analytes, enabling its desorption and ionization. As no organic matrix is required in this process, higher sensitivity of detection for small compounds is expected since no fragment from the matrix is produced. PSi has been already used for the realization of LDI-MS of small compounds by the group of G. Siuzdak and we have recently demonstrated that under UV laser illumination, the porosity of PSi plays a crucial role in energy transfer between PSi nanocrystallites and the analyte, significantly influencing its fragmentation and, consequently, the quality of the MS spectrum [38,39]. In a previous study, we fabricated silicon nanostructures decorated with Cu particles to achieve His-tag peptide enrichment with its subsequent MS detection with a sensitivity in the fmal range [31]. Each histidine residue carries on its side chain an imidazole group that can interact with Cu²⁺ ions present in the copper oxide (CuO). Here, we examined the ability of PSi-Cu surfaces to detect DMMP using SALDI-MS based on the fact that Cu²⁺ ions from CuO can form a strong coordination bond with P=O from organophosphate. To do so, the PSi-Cu surface, after its interaction with DMMP in the reaction chamber, was directly inserted in the mass spectrometer without any further treatment. Fig. 5 shows SALDI-MS spectra of DMMP obtained from PSi-Cu (porosity 80 %) after interaction with DMMP under the same experimental conditions as described above with 2 laser fluences (70 and 78 %). In Fig. 5A, three peaks are observed in the mass spectrum of DMMP: a peak at *m/z* = 186 corresponding to the copper adduct [M + Cu]⁺ with a signal-to-noise (S/N) ratio of 27 and additional peaks at *m/z* = 124 and *m/z* = 142, attributed to [M + H]⁺ and [M + H₂O]⁺, respectively [40]. Fig. 5B presents the MS spectrum of DMMP acquired under the same experimental conditions but with a slightly higher laser fluence (80 %). A strong increase in the intensity of the [M + Cu]⁺ signal is observed, with a S/N ratio reaching 131. This result is consistent with the findings of Sakurai et al. who demonstrated that CuO particles supported on zeolites efficiently promote ionization of small molecules such as amino acids via copper adduct formation [41].

Here, we demonstrated that PSi-Cu materials are capable of detecting the presence of DMMP by FT-IR and then confirmed its presence by SALDI-MS. The observed copper adduct confirms that DMMP strongly interacts with DMMP *via* coordination link. Such combinaison of FT-IR and LDI-MS techniques was already demonstrated by Smirnova et al., where they used TiO₂ porous materials to follow stearic acid photo-degradation [42]. We think that this type of dual surface is very innovative in the sense that if the precise assignment of the peak of interest can sometimes be difficult, especially when using FT-IR, the presence of

the molecule will be confirmed unambiguously by SALDI-MS.

## 4. Conclusion

The detection of pollutants such as OPs in air, soil, food or water has become a crucial public health issue, both for the specific populations that use these compounds (directly exposed) and for the general population, which is indirectly exposed. Thereupon, it is also essential to have effective analytical methods to assess the level of exposure from biological fluids (blood, urine, exhaled breath) or to detect their presence in indoor and outdoor ecosystems or environments, with the aim to quickly adapt and propose either an adequate therapeutic response or apply more sustainable habits. In this work, we used PSi structures decorated with Cu particles. To do so, we played with the PSi porosity, the type of copper salt and concentration, the presence of additives, the solvent composition and the deposition time to find the best experimental conditions. Porosity of 80 %, no PEG as additive, CuCl₂ as copper salt at 0.5 g/L and a deposition time of 60 s were found to be the best experimental conditions to form a reproducible PSi-Cu surface. Then, we succeeded to detect DMMP, a sarin simulant, by FT-IR, with the appearance of characteristic absorption bands in the mid-IR at 1246 cm⁻¹ (P=O) and 1319 cm⁻¹ (P-CH₃). Thanks to the ability of our PSi-Cu surface to also perform SALDI-MS detection, we proved that DMMP strongly interacts with Cu, with an MS peak observed at *m/z* = 186 corresponding to a copper adduct of DMMP [M + Cu]⁺. Finally, it should be noted that there are no reports mentioning the ability of modified PSi to detect DMMP molecule by FT-IR and SALDI-MS bimodally. Rather than aiming to optimize performance metrics such as detection limits or sensitivity, this study focused on demonstrating, for the first time, the feasibility of combining FT-IR spectroscopy and matrix-free SALDI-MS on a single copper-decorated porous silicon platform, without the need for any additional surface treatment. While FT-IR spectroscopy, a routine technique, may yield similar spectral features for various organophosphonates, SALDI-MS allows clear discrimination among them based on their distinct m/z, thus providing high selectivity and unambiguous identification between various CWA simulants. By integrating these techniques, we could obtain a more complete understanding of the composition, structure, and properties of complex samples, making it an invaluable tool in fields such as chemistry, pharmaceuticals, forensics, and environmental science.

### CRediT authorship contribution statement

**Warda Raiah:** Methodology, Investigation, Formal analysis.
**Mohammed Guendouz:** Writing – review & editing, Validation, Supervision, Methodology, Investigation, Formal analysis.
**Parastesh Piratesh:** Writing – review & editing, Validation, Methodology, Data curation.
**Vincent Thomy:** Writing – review & editing, Validation, Supervision, Methodology, Formal analysis, Data curation.
**Joël Charrier:** Writing – review & editing, Validation, Supervision, Resources, Project administration, Methodology, Funding acquisition, Data curation.
**Yannick Coffinier:** Writing – review & editing, Writing – original draft, Validation, Supervision, Resources, Methodology, Investigation, Formal analysis, Data curation, Conceptualization.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgements

The authors acknowledge the Agence Nationale de la Recherche (ANR) for financial support as part of the ANR MID-VOC project (ANR-17-CE09-0028-01), the Centre National de la Recherche Scientifique
(CNRS), the University of Lille and the University of Rennes. This work was partly supported by the French RENATECH network (French national nanofabrication platform) and the CMNF platform of IEMN.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.microc.2025.115034

### Data availability

Data will be made available on request.

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