Ultrasensitive Detection of Malondialdehyde with Surface Enhanced Raman Spectroscopy

Transcription

1 1 Analytical and Bioanalytical Chemistry Electronic Supplementary Material Ultrasensitive Detection of Malondialdehyde with Surface Enhanced Raman Spectroscopy Dongmao Zhang, Rukshani Haputhanthri, Siyam M. Ansar, Karthikeshwar Vangala, Andrzej Sygula and Svein Saebo

2 2 1: UV-vis spectrum of HPLC purified TBA-MDA Fig. S1: UV-vis spectrum of TBA-MDA adduct.

3 3 2: Raman spectrum of Ramchip slide and possible SERS interference of phosphoric acid to TBA-MDA spectrum acquired with TBA-MDA in acidic solution Fig. S2: (a) Raman spectrum of Ramchip slide, and SERS spectra of (b) AgNP aggregated with 1% KCl, (c) solution of 220 mm in H 3 PO 4 and 1% in H 2 SO 4, (d) 42 mm TBA in water, (e) ~ 10 µm HPLC purified TBA-MDA, (f) ~10 µm TBA-MDA in solvent that is 220 mm in H 3 PO 4 and 1% in H 2 SO 4. While the normal Raman spectrum was acquired with integration time of 100 s using laser power before objective 3.23 mw, the SERS spectra were acquired with the integration time from 1 to 10 s. The 239 cm -1 peak in spectrum (b) and (c) is from the Cl - adsorbed onto AgNP. The high similarity between spectra (b) and (c) indicate the acid solvent (220 mm H 3 PO 4 and 1% in H 2 SO 4 ) used in the TBA/MDA reaction mixture has no detectable SERS signal. Also evident from the comparison of spectrum (f) with (a), (c) and (f), there are no Ramchip substrate background, KCl and solvent acid spectral interference in the SERS spectrum of TBA-MDA obtained in the reaction solvent.

4 4 3. Density Function Calculations and Tentative Raman Peak Assignments To facilitate Raman peak assignment of TBA and TBA-MDA, a series of DFT calculations were performed using the PQS software package on a PQS computer [45, 46 ] The calculations included full geometry optimization without symmetry constraints as well as calculation of vibrational frequencies and Raman intensities. Tentative Raman peak assignments shown below were conducted for the normal Raman spectra obtained with TBA and TBA- MDA powder by correlating the experimental normal Raman spectra with the DFT Raman spectra calculated based on the procedure described in the experimental section (see main text for detail). It is important to point out that even though TBA, TBA-MDA are relatively small molecules, they are structurally very complicated as both molecules can adopt multiple tautomeric forms as shown in Fig. S3 and S4 in the Electronic Supplementary Material [48, 49]. While the most stable tautomeric forms in solid that we used for our DFT calculation can be found from the literatures [44, 47-49], pinpointing the correct tautomeric composition of TBA, TBA-MDA on the AgNP surface can be difficult and it is beyond the scope of current study. 3.1 Raman peak assignment for TBA: Normal Raman and SERS spectra of TBA were shown together with the DFT Raman spectra of TBA N1 and N10 forms, the two most likely tautomeric forms in TBA solid [47, 48]. Evidently the DFT Raman spectra calculated with both tautomeric forms have many overlapping Raman features and it is possible that both tautomeric forms were presented in the TBA powder used for the normal Raman acquisition. As a result, we conduct tentative Raman peak assignments based on the DFT Raman spectra calculated for both tautomeric forms and the results were shown in Table S1 and S2. Evidently conclusive peak assignments of the TBA Raman features require first identifying tautomeric form of the TBA using x-ray crystallographic data by growing single TBA crystal consisted of single tautomeric form, which is out of the scope of the current study. 3.2 Raman peak assignment for TBA-MDA: DFT Raman spectra of TBA-MDA were calculated for the two tautomeric forms known to be the most stable in TBA-MDA solid [44, 49]. Apparently compared to that obtained with tautomer 2, the DFT Raman spectrum of tautomer1 has significantly higher similarity to the experimental Raman spectrum of TBA-MDA, suggesting that the tautomer 1 is the dominant species in the TBA- MDA sample used for our Raman measurements. As a result, only the DFT Raman spectrum of tautomer 1 was used for the tentative Raman peak assignment shown in Table 3.

5 5 Fig. S3: Possible keto-enol tautomers of TBA[48] Fig. S4: The two most stable tautomeric forms of TBA-MDA in solid [49]

6 6 Fig. S5: (A) (a)normal Raman Spectrum of TBA powder, (b) SERS spectra of 42 mm TBA aqueous solution, and (B) DFT Raman spectra of TBA tautomer N1, (C) DFT Raman spectrum of TBA tautomer N10

7 7 Table S1: Experimental and DFT B3LYP/ G** Computed Vibrational Frequencies of TBA tautomer N1 (Abbreviated) a

8 8 Table S2: Experimental and DFT B3LYP/ G** Computed Vibrational Frequencies of TBA tautomer N10 (Abbreviated) a

9 9 Fig. S6: (A) (a) Normal Raman spectrum of TBA-MDA powder, and SERS spectra of (b) 10 µm HPLC purified TBA-MDA. DFT Raman spectral calculated for (B) tautomer 1, of TBA-MDA shown in figure S4 in Electronic Supplementary Material.

10 Table S3: Experimental and DFT B3LYP/ G** Computed Vibrational Frequencies of TBA-MDA adduct tautomer 1 (Abbreviated) a 10

11 11 4: SERS detection of TBA-MDA in TBA/MDA reaction mixture where the concentration of TBA is 0.7 mm. Fig. S7: SERS spectra of TBA/MDA reaction mixture where the concentration of TBA is 0.7 mm and concentration of MDA is (a) 3.3nM, (b) 33nM, (c) 0.3µM, (d) 3.3µM. Spectra was scaled and offset for clarity. TBA-MDA features remained dominant even when the concentration of MDA was as low as 3.3 nm.