SUPPORTING INFORMATION A New Approach for the Surface Enhanced Resonance Raman Scattering (SERRS) Detection of Dopamine at Picomolar (pm) Levels in the Presence of Ascorbic Acid Murat Kaya, Mürvet Volkan * Department of Chemical Engineering and Applied Chemistry, Atilim University, TR- 06800 Ankara, Turkey. Department of Chemistry, Middle East Technical University, TR-06531 Ankara, Turkey. * Corresponding author. Tel.:+903122103228; Fax: +903122103200; E-mail: murvet@metu.edu.tr 1
EXPERIMENTAL Materials: All chemicals were purchased from Aldrich unless otherwise stated and used without further purification: AgNO 3, Fe(NO 3 ) 3, dopamine hydrochloride ((HO) 2 C 6 H 3 CH 2 CH 2 NH 2 HCl), sodium citrate (HOC(COONa)(CH 2 COONa) 2 ) and AB- NTA (N α,n α -Bis(carboxymethyl)-L-lysine hydrate). Milli-Q water (Millipore) was used in all the experiments. All glassware used in the production of nanoparticles were cleaned with distilled water and ethanol and then dried in an oven at 50 C before use. Instrumentation: The structural, optical, and spectroscopic properties of prepared nanoparticles were examined using the following measurements. Field Emission Scanning Electron Microscope (FE-SEM) measurements were carried out with a Quanta 400 F field emission scanning electron microscope. Energy-dispersive X-ray analyses (EDX) were performed with a Quanta 400 F field emission scanning electron microscope equipped with energy-dispersive X-ray analyser. UV-Visible absorption data were recorded over a range of 400-900 nm with the UV-Vis spectrophotometer (UV-1601 PC, SHIMADZU). The iron content of the surface modified silver nanoparticles was determined by using a Leeman Direct Reading Echelle inductively coupled plasma optical emission spectrometer (ICP-OES). FT-IR measurements were performed by using Digilab FTS 3000 Excalibur series instrument in the mid-ir region. SERRS spectra were measured with a Horiba-Jobin-Yvon, model Lab Ram Raman micro spectrometer. It has a holographic grating having 1800 grooves mm -1 and a 632.8 nm HeNe laser with an exposure time of 10 s. The spectra were collected with Olympus model LMPlanFL, 10X, 50X and 100X microscope objectives. The laser power was approximately 10 mw and Raman signal was detected with a Peltier-cooled CCD camera. 2
Silver Nanoparticles: 500 ml of 1x10-3 M silver nitrate solution was boiled and then 10 ml of 1% sodium citrate solution was added to the boiling solution drop wise and allowed to continue boiling for 1 hour with vigorous stirring. Then it was brought to a final volume of 500 ml. The resulting colloids had yellow/greenish color. Surface Modification of Silver Nanoparticles with Iron-Nitrilotriacetic Acid Complex: 1 ml of 2x10-2 M Fe(NO 3 ) 3 solution was mixed with 1 ml of 2x10-2 M chelate (NTA) solution and the ph was adjusted to 7.0 by the addition of NaOH to achieve the iron nitrilotriacetic acid (Fe(NTA)) complex. After waiting 30 minutes, functionalization of silver nanoparticles with Fe(NTA) complex was performed by mixing 8 ml of Ag colloids with 2 ml of Fe(NTA) complex solution at room temperature and allowed to stand for 15 min. The colloidal solution of Ag-Fe(NTA) was centrifuged twice (15 min, 5000 rpm) to remove excess unbounded Fe(NTA) and then redispersed in 1 ml water and the ph was adjusted to 7.0 by the addition of NaOH. SERRS Studies: Various fresh stock solutions of analytes were prepared in water in varying concentrations and used for serial dilutions. The substrates for SERS measurements were prepared by simply dropping the 20 µl of prepared nanostructures onto glass slides which were cleaned with ethanol previously. Then substrates were used for the measurements of analytes directly or after allowed to dry. 3
RESULTS AND DISCUSSION The structural characterization of surface-modified AgNPs was performed with Field Emission Scanning Electron Microscope (FE-SEM) and the qualitative elemental composition of the prepared Fe(NTA)-modified AgNPs was identified with Energydispersive X-ray analyses (EDX). The results are shown in the Figure S1. Figure S1. a) FE-SEM image and b) EDX pattern of Fe(NTA) modified AgNPs (diameter= 31±5 nm). Identification and quantification of Fe(NTA) binding on AgNPs were also performed by inductively coupled plasma optical emission spectrometer (ICP-OES). The initial concentration of Fe(NO 3 ) 3 was 1120 mg/l in the mixture. After complex formation, Fe(NTA) was bound to AgNPs and then Fe(NTA)-modified AgNPs were centrifuged and washed several times. The remaining parts were combined and analyzed with ICP-OES. 4
The ICP-OES measurements were carried out and the average amount of bound Fe on the surface of the AgNPs was found to be 520±20 mg/l (10 replicates). Functionalization of nanoparticles with NTA groups was confirmed by FT-IR measurements. The FT-IR spectra of the pure NTA and Ag-Fe(NTA)-assembly are shown in Figure S2. Figure S2. FTIR spectra of (A) AB-NTA (N α,n α -Bis(carboxymethyl)-L-lysine hydrate) and (B) Fe(NTA) modified AgNPs. 5
The FTIR spectrum of Fe(NTA)-modified AgNPs displayed characteristic vibrational modes associated with the NTA carboxylate groups. The bands at 1614 cm 1 and 1385 cm 1 were assigned to the asymmetric and symmetric stretching vibration bands of the deprotonated carboxylic acid group of NTA. Figure S3. SERRS signal of 1x10-5 M DA acquired with Ag-Fe(NTA) nanoparticles as substrate A) the day, B) after 5 days, C) after 7 days, D) after 15 days of their preparation. 6
The SERRS spectra of 1.0x10-5 M DA was measured utilizing Ag-Fe(NTA) nanoparticles as substrate either directly or in dry conditions. The results are given in Figure S4. As can be deduced from Figure S4, SERRS signal height acquired from wet droplet was twice of that obtained from the dried one. So signal acquisition from wet droplet was chosen and applied throughout the study. Figure S4. Comparision of SERRS signal of 1.0x10-5 M DA obtained with (A) wet and (B) dry conditions. 632.8 nm radiation from a Helium Neon laser was used with an excitation power of 10 mw and spectra was acquired with 10 s integration time. 7
Figure S5. UV-Vis spectra of A) aqueous Fe(NTA) complex, B) silver colloid and C) Fe(NTA) decorated silver colloid after loaded with dopamine. 8