Copyright WILEY-VCH Verlag GmbH & Co. KGaA, 69469 Weinheim, Germany, 2018. Supporting Information for Small, DOI: 10.1002/smll.201801523 Ultrasensitive Surface-Enhanced Raman Spectroscopy Detection Based on Amorphous Molybdenum Oxide Quantum Dots Hao Li, Qun Xu,* Xuzhe Wang, and Wei Liu
Copyright WILEY-VCH Verlag GmbH & Co. KGaA, 69469 Weinheim, Germany, 2016. Supporting Information Ultrasensitive Surface Enhanced Raman Spectroscopy Detection Based on Molybdenum Oxide Amorphous Quantum Dots Hao Li, Qun Xu*, Xuzhe Wang, Wei Liu Section 1. Experimental Section. Section 2. Theory Calculation. Section 3. Supplementary Figures. Figure S1. XPS spectra showing S 2p core level peak regions for a) H x MoO 3 dots and b) MoS 2 nanosheets after 2h sonication. Figure S2. a) Photographs of MoS 2, MoO 3 and H x MoO 3 quantum dots and b) their UV-Vis-NIR absorbance spectra. Figure S3. DLS measurements of H x MoO 3 dots under a) 11Mpa; b) 14 MPa; c) 17MPa; d) 17MPa; e) 20MPa SC CO 2. Figure S4. The size distribution calculated from TEM image under 17 MPa SC CO 2. Figure S5. TEM images of as-prepared sample fabricated under a) 11MPa SC CO 2 and b) 20MPa SC CO 2. Figure S6. XPS spectra showing Mo 3d core level peak regions for varied samples fabricated under a) 11 MPa; b) 14 MPa; c) 17 MPa; d) 17 MPa; e) 20 MPa SC CO 2. Table S1. Amount of substance and kinetic energy of CO 2 molecules under different SC CO 2 pressure. Table S2. Valence state distribution of Mo 3d under different SC CO 2 pressure. Table S3. SERS property comparison with previous works. Section 4. The calculation equation of Enhancement Factor.
1. Experimental Section. Materials. MoS 2 powder was provided by Sigma-Aldrich (Fluka, product number 69860). Hydrogen peroxide (H 2 O 2 ) 30% aqueous solution was purchased from Sinopharm Chemical Reagent Co., Ltd. (China). Ethanol in analytical grade was purchased from Sinopharm Chemical Reagent Co., Ltd. (China) and directly used owing to its analytical grade. CO 2 with a purity of 99.99% was purchased from the Zhengzhou Shuangyang Gas Co. Deionized water was prepared with double-distilled water. Preparation of amorphous MoO 3 quantum dots. A general fabrication process is as follows. MoS 2 powder (50 mg) was dispersed into the aqueous solution (10 ml) with an ethanol fraction of 30%. The mixed solution was sonicated for 2 h to make it have a good dispersity. Then 30% H 2 O 2 (1 ml) was mixed with the prepared solution (9 ml) to form a reaction system solution. The mixed solution was quickly transferred into the supercritical CO 2 apparatus composed mainly of a stainless steel autoclave with a heating jacket and a temperature controller. The autoclave was heated to 40 and CO 2 was injected into the autoclave until to a required pressure (20 MPa). Then CO 2 was released by a placid speed after 3h. Subsequently the reacted solution was centrifuged at 6000 rpm for 15 min to remove aggregates, and the supernatant was collected for the further reaction and characterization. Different contrast experiments were carried out under the different experiment conditions, such as the varied SC CO 2
pressure, H 2 O 2 fraction, etc., for the purpose to obtain the optimum experimental condition. Optical measurements. The as-prepared solutions fabricated under different conditions were irradiated under sufficient sunlight from 11:00 am to 16:00 pm (5 hours). Then the obtained solutions were characterized with UV-vis-NIR spectra to obtain their absorption spectra. Characterization. The morphologies of the obtained quantum dots were characterized by TEM (JEOL JEM-2100) and HRTEM (JEOL JEM-2100F). Raman measurement was carried out using LabRAM HR Evolution with laser wavelength of 532 nm. X-ray powder diffraction (XRD) analysis was conducted using an Ultima IV instrument. XPS analysis were performed using ESCLAB 280. UV-vis-NIR spectra were measured to evaluate the light adsorption using Shimadzu UV-240/PC. Size measurement was performed by Malvern Zeta sizer Nano ZS90 (Malvern, UK). Three kinds of dyes preparation and SERS test. Raman spectra of different kinds of probe molecule deposited on H x MoO 3 quantum dots samples as substrates were characterized under laser excitation at 633 nm. Specifically, MB, Rh6G and RhB aqueous solutions with concentration varied from 10-3 to 10-9 M were obtained from a primal solution of 10 2 M by dilution, respectively. Then 1mL of probe molecule solution with given concentration was mixed with 500 μl of H x MoO 3 aqueous
dispersion (1 mg ml 1 ) followed by 2 h storage in dark for an adsorption equilibrium. Finally, 20 μl suspension was extracted and dropped onto a cleaned silicon wafer (3mm 3mm) followed by drying at 60 C. Raman spectra of all samples were measured under same conditions afterwards. Raman measurement parameters setting: laser wavelength: 633 nm; power: 0.5 mw; lens: 50 long distance objective; acquisition: 10 s.
2. Theory calculation. Considering supercritical CO 2 doesn t belong to ideal gas, Van der Waals equation of non-ideal gas is given by equation 1 to describe the relation of conventional thermodynamic parameters in supercritical fluid. ( a 2) ( - ) (1) Here, a (0.364 Pa m 6 mol -2 for CO 2 ) and b (4.27 10-3 m 3 mol -1 for CO 2 ) represent two different correction factors [1]. In order to explore the fluid state in the reaction system more intuitively, a Van der Waals correction equation is carried out as equation 2 shows [2]. ( ) ( - ) (2) Upon this, P * =P/P c is the ratio of actual pressure P and critical pressure P * (7.28 MPa), V * =V/V c is the ratio of actual volume V and critical volume V * (94 cm 3 mol -1 ), T * =T/T c is the ratio of actual temperature T and critical temperature T * (304.2 K). Actual pressure P=17 MPa and actual temperature T=313.2 K are substituted into this modified equation to get an actual volume of 72.95 cm 3 mol -1. Then 0.68 mol CO 2 is worked out by a 50 ml autoclave. After that the molecular number under 17 MPa is estimated by Avogadro constant accurately, yielding a value of 4.12 10 23. Furthermore, the molecule average kinetic energy in this closed system could be obtained according to equation 3 as follows: (3) Where ε k is the molecule kinetic energy and k is the boltzmann constant.
3. Supplemental Figures Figure S1. XPS spectra showing S 2p core level peak regions for (a) H x MoO 3 dots and (b) MoS 2 nanosheets after 2h sonication.
Figure S2. (a) Photographs of MoS 2, MoO 3 and H x MoO 3 quantum dots; (b) their UV-Vis-NIR absorbance spectra.
Figure S3. DLS measurements of varied samples fabricated under (a) 11 Mpa; (b) 14 MPa; (c) 17 MPa; (d) 20 MPa SC CO 2.
Figure S4. The size distribution calculated from TEM image under 17 MPa SC CO 2.
Figure S5. TEM images of as-prepared sample fabricated under a) 11 MPa SC CO 2 and b) 20 MPa SC CO 2.
Figure S6. XPS spectra showing Mo 3d core level peak regions for varied samples fabricated under (a) 11 Mpa; (b) 14 MPa; (c) 17 MPa; (d) 18 MPa; (e) 20 MPa SC CO 2.
Table S1. Amount of substance and kinetic energy of CO 2 molecules under different SC CO 2 pressure. Pressure Amount of Substance Kinetic Energy 11 MPa 0.5910 mol 2.3073 10 3 J 14 MPa 0.6420 mol 2.5064 10 3 J 16 MPa 0.6716 mol 2.6223 10 3 J 17 MPa 0.6854 mol 2.6743 10 3 J 18 MPa 0.7157 mol 2.7943 10 3 J 20 MPa 0.7232 mol 2.8236 10 3 J
Table S2. Valence state distribution of Mo 3d under different SC CO 2 pressure. Pressure Mo 4+ Mo 5+ Mo 6+ 11 MPa 27.61 72.29-14 MPa - 24.83 75.17 17 MPa - 25.62 74.38 18 MPa - 20.99 79.01 20 MPa - 19.06 80.94
Table S3. SERS property comparison with previous works based on semiconductors. Substrate LOD (M) EF Substance Literature source MoS 2 10-7 Rh6G W 8 O 49 10-7 3.4 10 5 Rh6G α-zno 6.62 10 5 4-MPY, 4-MBA, 4-ATP MoO 2 10-7 3.75 10 6 BPA, DCP, PCP α-moo 3-x 10-8 1.8 10 7 4-MBA, Rh6G, MB TiO 2 10-6 MB Cu 2 O 10-9 8 10 5 Rh6G, CV Nature Communications. 2017; 8: 1993. Nature Communications, 2015, 6 (6, 7) :7800 Angewandte Chemie International Edition, 2017, 56 (33) : 9851 9855 Nature Communications, 2017, 8 :14903 The Analyst, 2016, 142 (2) :326 Journal of the American Chemical Society, 2014, 136 (28) :9886-9889 Adv. Mater. 2017, 29, 1604797 10-9 9.5 10 5 MB H x MoO 3 10-8 5.5 10 4 Rh6G This work 10-8 5 10 4 RhB
4. The calculation equation of Enhancement Factor. 1. The Raman signal data for MB (10-2 M), Rh6G (10-3 M) and RhB (10-3 M) were used as non-sers-active reference, respectively. Specifically, the intensity was obtained by taking average from measurements of several spots, and the EFs were calculated as the following equation [1] : I SERS is the intensity of a vibrational mode in the surface-enhanced spectrum; I Bulk the intensity of the same mode in the Raman spectrum; N SERS is the number of molecules sampled in the bulk; N Bulk is the number of molecules adsorbed and sampled on the SERS active substrate; C SERS is the concentration of solution sampled on the SERS-active substrate; C Bulk is the concentration of bulk solution. C is the molar concentration of the probe molecule solution, V is the volume of the droplet, N A is Avogadro constant. S Raman is the laser spot area μm in diameter) of Raman scanning, and S Sub is the area of the substrate (3 mm 3 mm). This equation is based on the assumption that the probe molecules were distributed uniformly on the substrates.
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