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The Role of TiO2 Nanoparticles and UV Irradiation on Enhancement of SERS Spectra Improving Levamisole and Cocaine Detection on Au Substrates Veronika Skoupá, Adéla Jeništová, Vladimir Setnicka, and Pavel Matejka Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b00358 • Publication Date (Web): 06 Mar 2019 Downloaded from http://pubs.acs.org on March 7, 2019
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The Role of TiO2 Nanoparticles and UV Irradiation on Enhancement of SERS Spectra Improving Levamisole and Cocaine Detection on Au Substrates Veronika Skoupáa*, Adéla Jeništováb, Vladimír Setničkaa, Pavel Matějkab aDepartment
of Analytical Chemistry, Faculty of Chemical Engineering, University of Chemistry and Technology Prague, Technická 5, Prague 6 - Dejvice, 166 28, Czech Republic bDepartment of Physical Chemistry, Faculty of Chemical Engineering, University of Chemistry and Technology Prague, Technická 5, Prague 6 - Dejvice, 166 28, Czech Republic *Corresponding author e-mail address:
[email protected]; Tel.: + 420 220 444 091 Keywords: Surface-enhanced Raman scattering; illicit drug; self-cleaning effect; spectral signal amplification; photodegradation effect, adulterants Abstract The original goal of this study was the employment of Surface-enhanced Raman spectroscopy (SERS) for analysis of real cocaine samples (containing adulterants) on composite Au-TiO2 nanomaterials to achieve low detection limits suitable for analysis of illicit drugs and controlled substances and to exploit the photodegradation activity of TiO2 to recycle the SERS substrate for repeated analyses. The photodegradation (self-cleaning) effects of the AuTiO2 composite nanomaterials by ultraviolet (UV) radiation are known. These effects were investigated on large-area SERS substrates immersed in the TiO2 nanoparticle aqueous suspension. The cocaine samples were measured on electrochemically gold-plated platinum targets. Surprisingly, the intensity of SERS spectra of the pure cocaine did not change after immersion in a suspension of TiO2 under UV irradiation. However, for some real cocaine samples, the overall intensity of the SERS spectra was even higher after the treatment by TiO2 and UV radiation as compared to the usual Au substrate. This unexpected signal amplification (valuable for illicit drug detection) was found to be caused mainly by the contained levamisole, which is used as a medical drug and is one of the frequent adulterants of cocaine. Both the sole effect of TiO2 on the levamisole spectrum intensity and the role of UV irradiation were inspected separately. Finally, an investigation of both the TiO2 and UV ACS Paragon Plus Environment
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radiation treatments was performed demonstrating (i) the necessity of both factors for selective SERS signal enhancement of the adulterant and (ii) revision of general anticipation of TiO2 role in SERS systems. Introduction Raman spectroscopy is an analytical method with important analytical benefits such as measurements in glass vessels or thin plastic containers of aqueous/humid samples. The poor detection limits of normal Raman spectroscopy are overcome by surface-enhanced Raman spectroscopy (SERS) with a detection limit reaching as low as ca. 10-9 mol⋅L⁻¹ on appropriate SERS-active substrates with an excellent localized surface plasmon resonance (SPR) effect [1, 2]. Briefly, the giant enhancement of the electromagnetic field is based on a confined collective quantum oscillation of the surface charges at the nanostructured surfaces of coinage metals (Au, Ag, Cu) [3]. Illicit drugs are a global problem; cocaine production is relatively simple yielding a product of high quality and purity. The adulterants in real samples of cocaine can be saccharides (e.g. glucose, sucrose or lactose), talc, starch, carbonates or many other compounds [4, 5]. The number of undesirable identified adulterants increased in last decades. Nowadays, the predominantly adulterants in real cocaine samples are levamisole, caffeine, phenacetin, lidocaine, diltiazem, hydroxyzine and procaine [6]. The possibilities of the SERS spectra collection of illicit drugs were studied using adhesive tapes [7]. The detection limits were examined depending on the nature of the plasmonic substrate [8]. According to the spectra themselves, it is difficult to identify the individual drug samples. For the identification of real samples the use of the multivariate statistical method namely the exploratory Principal Component Analysis (PCA) is suggested. It can distinguish individual illicit drugs and can be used for drug profiling [9].
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Since the materials for SERS-active substrates preparation are expensive, the efforts to recycle them are growing. In the last decade, the combination of metal plasmonic nanomaterials and TiO2, ZnO or Fe3O4 nanoparticles (NPs) was studied frequently [10-21]. Sinha et al. [11] prepared SERS-active material from ZnO nanotubes coated by an Au layer. ZnO contributes to the SERS signal enhancement and is also used for its high refractive index. After the SERS spectra measurements, the whole substrate was treated by UV radiation, which led to the analyte’s degradation. The process resulted in the surface regeneration and the SERS-active material can be used for modification by another analyte. The major representatives of photocatalytic substances include TiO2, which is important for its chemical stability, relative nontoxicity and fairly low purchase price [12]. The composite photocatalysts consisting of TiO2 and noble metal nanostructures are considered to be suitable and eminent materials for increasing the efficiency of the photocatalytic (mainly photodegradation) process performed in both the liquid and gas phases [13]. It is obtained that the irradiation of the TiO2 surface modified by metal NPs leads to the (plasmonic/electronic) excitation of the metal NPs. The metal NPs can be characterized by localized SPR profile [14]. This effect was used by several groups for comparing studies of the SERS spectra enhancement between Au NPs and Au-TiO2 composite NPs [15-20]. The SERS spectra of 4mercaptobenzoic acid (4-MBA) absorbed on the pure TiO2 NPs were compared with the SERS spectra of 4-MBA on Au-TiO2 NPs. The SERS spectra measured on Au-TiO2 NPs exhibited higher intensities, which probably depend on an enhanced effect inducted by a charge-transfer with the interaction/synergic effect of the TiO2 and Au components of the substrate. The charge-transfer from TiO2 to the molecule and other charge-transfers supported by localized SPR are responsible for the significant SERS spectra enhancement of molecules on the Au-TiO2 NPs. Moreover, Au-TiO2 NPs are known for their high stability and recyclability, which make them active SERS substrates for different molecules [15]. The
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electromagnetic field generated by metal NPs interacting with UV irradiated TiO2, leads to a further increase in the number of charge carriers on the surface. This phenomenon can thus contribute to the material photocatalytic properties [16, 17]. TiO2 porous microspheres with noble metal NPs under UV irradiation were investigated as highly active photocatalysts for the removal of toluene from the gas phase or phenol from the liquid phase. Nevertheless, the dosage and size of the noble metal NPs were the major factors influencing the photoactivity of the modified TiO2 microspheres [13]. The degradation by the combination of UV radiation and metal-TiO2 nanostructures was investigated for methylblue and methylorange [18], Acid Red 88 [19] or for gaseous volatile organic compounds [20]. In this study, we show that the degradation of organic compounds under UV irradiation in the composite systems with plasmonic metals and TiO2 NPs is not a general phenomenon. Moreover, we demonstrate the signal enhancement in surface-enhanced Raman spectra after the UV light treatment. This effect (requiring both TiO2 NPs and UV radiation) depends on the type of organic compound and influences the relative band intensities of the individual components in real mixtures.
Experimental Chemicals Potassium tetrachloroaurate (KAuCl4, 99.995%, Sigma-Aldrich, CAS: 13682-61-6); ultrahigh purity water purified using a Milli-Q® system (H2O); aluminium (III) oxide (Al2O3, ≥ 98%, Sigma-Aldrich, CAS: 1344-28-1); calcium carbonate precipitated (CaCO3, p.a., PENTA, CAS: 471-34-1); hydrogen peroxide (H2O2, 30%, p.a., PENTA, CAS: 7722-84-1); sulphuric acid (H2SO4, 96%, p.a., PENTA, CAS: 7664-93-9); suspension of titanium dioxide NPs (TiO2, 99,9%, Sigma-Aldrich CAS: 13463-67-7; caffeine hydrochloride (99%, FAGRON, CAS:
15E06-T20-029602);
levamisole;
cocaine
standard
(hydrochloride,
≥99.5%,
pharmaceutical grade, Dr. Kulich Pharma) and a real sample of cocaine K-770 (obtained from
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the Police of the Czech Republic). The composition of K-770 sample and the purity cocaine standard was analysed by GC-MS, LC-MS and NMR spectra in the Forensic Laboratory of Biologically Active Substances of the University of Chemistry and Technology, Prague. The sample K-770 contained 62.5 % (m/m) of cocaine and the second major component was levamisole. The other minor substances were not quantified. Preparation of SERS-active substrate and samples Firstly, a platinum target (cylindrical shape, diameter 8 mm, height 1.5 mm, Safina, Inc., Fig. 1S) was abraded with sandpapers no. 3, 5 and 7, then Al2O3 and CaCO3 were used to regenerate the Pt surface. Immediately, the target was rinsed with in H2O and immersed in a solution of H2SO4 and H2O2 (3:1). After 15 minutes, the target was taken out from the solution, rinsed with H2O and dried. The cleaned Pt target was gold-plated in a tetrachloroaurate bath by cathodic reduction as described [22]. For the first 5 minutes, the direct current was set to 5 mV. For the next 5 minutes, the current was set to 10 mV and the last 5 minutes was at 15 mV to obtain a SERS-active surface. The gilded target was rinsed with with H2O and immersed in 1 mL of the individual sample solution. The samples were represented by (i) a real sample of cocaine, (ii) cocaine standard, (iii) levamisole, and (iv) caffeine. The same mass concentration of 10-3 g⋅L⁻¹ was used for each of the samples for mutual comparison of the results. We did not use the molar concentration, because (i) it was not possible to calculate the molar concentration values for the real sample (it is a mixture where the exact composition is unknown) and (ii) the mass concentration values reflect somewhat the size of molecules important for the surface coverage. (The same molar concentration of molecules of different size represents very different surface coverage.) After 12 hours, the SERS-active target with the deposited molecules was taken out of the individual solution, washed with H2O and dried.
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Instrumentation and measurements The SERS spectra were measured on a DXR Smart Raman spectrometer (Thermo Fisher Scientific, USA) equipped with the Peltier-cooled CCD (charge-coupled device) detector and two diode lasers emitting at 780 nm and 532 nm using the 40-mW and 10-mW excitation laser power, respectively. 10 s acquisition time was repeated as 10 scans. The collected spectra (Stokes region from 3500 to 50 cm-1) were afterwards automatically baselinecorrected in the Omnic 8.1 software (Thermo Scientific, USA). Moreover, supplementary experiments were performed using the Raman spectrometer with microscope adapter (Renishaw InVia Reflex, GB) equipped with HeNe laser (633-nm excitation) and Peltier cooled charge-coupled device (CCD) detector. Microscope adapter was equipped with 5 ×, 20 ×, 50 × and 100 × zoom objectives. The spectra were collected with 10 % laser power (max. 20 mW for 633 nm laser excitation), 5 accumulations of 1 scan and 10 s of exposure time acquired under control of the software WiRETM (Renishaw, GB). Ultraviolet-visible (UV-Vis) spectra were recorded in 5-mm Suprasil 300 quartz-glass cuvettes (examined volume 1 mL) (Hellma Analytics, Germany) using a CINTRA 404 true double beam absorption spectrometer (GBC, United Kingdom) in the range of 200–800 nm at a speed 300 nm⋅min⁻¹. The morphology of SERS substrates was investigated using scanning electron microscopy (SEM) Lyra equipped with dual beam microscope and a FEG electron source (Tescan, Czech Republic). To conduct the measurements, the samples were placed on a carbon conductive tape. The SEM measurements were carried out using a 10-kV electron beam. The individual SERS-active substrate (cylindrical target) with the deposited molecular sample was measured on a DXR Raman spectrometer at 10 different (randomly selected) points. The target was then immersed in 10 mL of TiO2 NPs (1g⋅L⁻¹) [23] suspension and stirred under
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polychromatic UV light (the emission maximum of the broad emission UV lamp was ca. 255 nm with an output power of 75 W). The irradiation time was optimized to 1 h. At shorter irradiation times, the spectral intensity was lower, longer times more than 2 h may lead to the analyte thermal desorption. After 1 h, the target was removed from the suspension, rinsed thoroughly with H2O, dried by a stream of gaseous N2. Subsequently, the treated sample was examined on a DXR Raman spectrometer at 10 different points again under the conditions described above. The mean spectra and the spectral standard deviation records were calculated for each set of 10 spectra for mutual comparison of treated samples with corresponding untreated ones. Data processing and evaluation The UV-vis spectra and SEM images were used without any data processing. The Raman spectra were baseline-corrected, labeled and averaged using the OMNIC 8.1 software (Thermo, USA). The peak heights and bands areas were calculated using the same software; the values were evaluated using Excel (Microsoft, USA). Results and discussion The intended goal of this study was originally to clean the SERS-substrates by combined treatment with TiO2 suspension and UV irradiation. To check the role of the plasmon resonance enhancement of Raman signal, three different excitation wavelengths were tested. The acceptable SERS signal was obtained only at 633 and 780 nm excitation wavelengths. In the case of 532 nm, no spectral features above the noise level were observed, approving the plasmon resonance of gold substrate occurs at longer wavelengths [24]. The best band’s resolution was obtained at 780 nm excitation. Hence, the SERS spectrum of K-770 and SERS spectrum of the same sample after 1-hour treatment in TiO2 NPs suspension under UV light are compared in Figure 1. It has been expected that the intensity of the whole SERS spectra
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decreased after the described treatment due to photodegradation mediated by TiO2 NPs (considering the absorption of the samples in the UV range). On the contrary, a significant intensity increase of SERS spectra is evident by a number of vibrational bands. Thus, the new goal of the study was to elucidate this unexpected observation. We should notify, that all examined samples are colorless substances; thus, they do not exhibit absorption and the consequent fluoresce in visible and NIR regions, i.e. at the Raman scattering excitations used. Hence, it was not necessary to filter any fluorescence signal in SERS/Raman spectra.
Fig. 1: SERS spectra of the real cocaine sample K-770 before UV irradiation and without TiO2 suspension (a) and after 1 hour of UV irradiation in TiO2 NPs suspension (b) (780 nm excitation wavelength)
Considering the results of the LC-MS and GC-MS analyses of the sample of K-770 (see the experimental part), two main components of K-770 are cocaine and levamisole. Hence, their pure reference samples were examined by SERS spectroscopy under the same experimental conditions as the K-770 and the SERS-spectra amplification after TiO2 and UV light treatment was observed similarly to real sample. The comparison of the amplified SERS
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spectra of K-770, cocaine standard and levamisole (after TiO2 and UV light treatment) are shown in Figure 2. The amplified Raman bands in the spectra of the real K-770 sample correspond mainly to the amplified SERS spectrum patterns of levamisole (Raman shifts 2937, 2141, 1547, 1413, 1273, 1000 cm-1). The band intensities of the cocaine standard were significantly lower (but amplified compared to data without TiO2 and UV light treatment). The majority of the vibrational bands of the cocaine were overlapped by amplified bands of levamisole (Raman shifts 2928, 2128, 1598, 1001 cm-1) (Fig. 2S). Nevertheless, the bands at 1716 and 891 cm-1 were observed only in the spectrum of cocaine (see supplementary materials).
Fig. 2: Amplified SERS spectra of the real cocaine sample K-770 (a), levamisole standard (b) and cocaine standard (c) (obtained after 1 hour of UV irradiation in TiO2 NPs suspension at 780 nm excitation wavelength)
Because the K-770 sample is a complex mixture and the known components exhibit overlapping features, it was impossible to assign the all vibrational bands to the individual substances. Hence, we examined and interpreted further the SERS of the standard of cocaine
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and the standard of levamisole. The SERS spectra of the cocaine standard before and after the influence of TiO2 NPs and UV light are shown in Figure 3. In this case, the intensity of the SERS spectra was not changed apparently for most of the bands. Some bands increased, any band intensity did not decrease significantly, which means the cocaine did not decompose. Some differences of the band intensities/areas (both increases and tiny decreases) and namely the narrowing of some bands (2931, 2128, 1717 cm-1) indicate partial rearrangement of the adsorbed cocaine molecules. Hence, the photodegradation effect cannot be used in the case of absorbed cocaine to renew/clean the Au SERS-active surface.
Fig. 3: SERS spectra of the cocaine standard before the influence of TiO2 NPs and UV light (a) and after 1-hour exposition (b) (780 nm excitation wavelength)
In the SERS spectrum of the cocaine standard, the characteristic bands of aliphatic skeletons at 2931 cm-1 (-CH2- stretching vibration) and deformation vibration δ(CH) coupled with ν(CC) at 1350 cm-1 were observed. The band at 2128 cm-1 belonged to the tertiary amine hydrohalide. The vibrations at 1537 and 1327 cm-1 were characteristic for δ(CH3) of CH3-N
