Tri-sodium citrate dihydrate (Merck), gold(III) chloride (30 wt% in HCl, Sigma- Aldrich), europium chloride hexahydrate (Sigma), 1,10-phenanthroline
Chapter 4 monohydrate (Merck), 4-mercaptobenzoic acid (Sigma-Aldrich), sodium
hydroxide (Merck) were used as received. Milli-Q grade water (18.2 MΩcm, Millipore) was used for synthesis as well as for other experimental work.
4.3.2. Synthesis of [Eu(Phen)
2(MBA)
2]Cl
In a typical reaction, phenanthroline (20.7 mg, 0.1044 mmol) was dissolved in 12 mL methanol to which 4-mercaptobenzoic acid dissolved (with 1M, NaOH) in 4 mL methanol (16.4 mg, 0.1063 mmol) was added sequentially at 60 °C under stirring condition. After 5 min, 4 mL methanolic solution of EuCl3.6H2O (20.7 mg, 0.0565 mmol) was added dropwise under continuous stirring condition resulting in milky white appearance. The mixture was refluxed at 60 °C for 3 h, during which the milky white suspension turned into light orange dispersion. The dispersion was allowed to cool to room temperature and was filtered to remove any insoluble material. The filtrate so obtained was solvent dried and excess water was added to precipitate the complex. The precipitate so obtained was then dried in dessicator (following decantation of water) overnight to obtain solid orange powder and was then used for further characterization.
4.3.3. Synthesis of citrate stabilised gold nanoparticles (cit-Au NPs)
In a typical synthesis, 100 mL aqueous solution containing 50 mg tri-sodium citrate was refluxed at 135 °C with continous stirring for 30 min. At the boiling condition, 1 mL of HAuCl4 (1.72 ×10-2 M) was then added and the reaction was further allowed to continue for next 30 min. The formation of cit-Au NPs was ensured by appearance of deep red color dispersion, which was further characterised by UV-Vis and TEM studies.
4.3.4. Synthesis of gold nanoparticle (Au NP) dimer
1 mL of as-synthesized cit - Au NP dispersion was diluted in 5 mL aqueous solution. To the above dispersion, 50 μL (30 μM) methanolic solution of [Eu(Phen)2(MBA)2]Cl was added dropwise, with vigorous stirring at room temperature. The resulting mixture was stirred for 2 min to obtain metal complex ligated Au NP dimer. Similarly, the reaction was carried at two other different
concentrations of metal complex viz., 15 μM and 90 μM. In each 5 mL aqueous dispersion of Cit - Au NPs, 50 μL (15 μM or 90 μM) of metal complex was added dropwise and stirred vigorously for 2 min under the same condition. The resulting NP dispersion was then centrifuged at 20267 rcf for 10 min; the pellet was collected and was finally redispersed in 5 mL water for storage and further use.
4.3.5. Sample preparation for normal Raman and SERS experiment
SERS measurements were carried out in 1.7×1.7 cm2 pre-cleaned oven-dried glass slides. The glass slides were cleaned following three-step cleaning process: The glass slides were first dipped in piranha solution (H2SO4/H2O2) for 30 min and then rinsed with Milli-Q water. Those were then transferred to acetone solution and sonicated for 30 min. That was followed by repeated washing of the glass slides with Milli-Q water, which was then oven-dried at 60 °C for further use. For all Raman measurements, 100 uL of the sample was dropcast on the clean glass slide to make a thin film of it, which was dried in vacuum dessicator. The Raman spectra for the samples were then recorded in a LabRam HR Raman instrument using 532 nm and 633 nm laser source as per requirement. All measurements were carried out with 100× objective with an acquisition time of 20 s.
4.3.6. Tip enhanced Raman spectroscopy (TERS) experimental set-up
The TERS set-up consists of Bruker’s Innova- IRIS (Integrated AFM Raman Imaging system) system integrated with LabRAM HR evolution Raman (Horiba Jobin Yvon) spectrometer using an optic coupler that focuses laser into the scanning probe microscope (SPM, Bruker) probe. For SPM-TERS operation, a sharp gold metallic tip (Bruker) was used. At first, STM or SFM gold tip was engaged to obtain topographic information from the region of interest (ROI) that was pre-identified using optical microscope. TERS acquisition was then carried out using a side illumination optical set-up. A long working distance objective (Olympus, 50x, NA=0.42) placed at 60° angle with respect to the tip-axis was used to focus the He-Ne (632.8 nm) laser at the tip-apex. The Raman signal was then collected in back-scattered mode through the same objective, which was sent to
Chapter 4 Horiba Jobin Yvon spectrograph, at 600 lines per mm grating. TERS acquisition
was done employing two types of SPM technology: scanning force microscopy (SFM)-TERS and scanning tunnelling microscopy (STM)-TERS mode. For carrying out TERS, we have assumed that a single complex had ligated Au nano-dimer through the Raman active 4-MBA. The enhanced electric field at the SPM tip-apex when aligned with 632.8 nm laser allowed us to achieve Raman spectral information from individual Au nano-dimer at the spatial resolution of less than 100 nm.
(a) SFM-TERS: SFM-TERS set up consisted of a fine gold probe (with tip radius of about 20 nm) attached to quartz tuning fork, which was mounted on a tuning fork cartridge that included a piezoelectric element to induce oscillation of the tuning fork.
Sample preparation: SFM-TERS measurements were done on pre-cleaned silicon substrate. The silicon substrate was first sonicated in piranha solution (H2SO4/H2O2) for 30 min and then rinsed with Milli-Q water. This was then rinsed in acetone solution (thrice) followed by repeated washing with Milli-Q water, which was then dried overnight at room temperature for further use. The as- synthesized Au nanodimer dispersion (~ 8 μL) was dropcast on the pre-cleaned silicon substrate and was then allowed to dry overnight.
Imaging condition: The SFM images were obtained by engaging the gold tip with the sample at set point value of 4.5 V. The scan was then performed in tuning fork amplitude feedback mode. TERS acquisition was then carried out with 632.8 nm laser (ND filter = 25 %) focussed at the gold tip-apex with an acquisition time of 20 s.
(b) STM-TERS: The STM-TERS set-up involved a solid gold etched wire and a conductive substrate and sample for operation at a constant current feedback loop.
Sample preparation: For STM-TERS sample preparation, 8 μL as-synthesized Au nanodimer dispersion was dropcast on carbon coated copper TEM grid, which was then allowed to dry overnight. The dried copper TEM grid was then placed on the sample holder by applying silver paste on the edge of the TEM grid to generate conduction between the sample and the substrate.
Imaging Condition: The STM-TERS operation was conducted by engaging the etched gold tip at a set-point current of 1.0 nA and a bias voltage of 0.5 V. For TERS acqusition, 632.8 nm laser was aligned with the gold tip-apex and spectral acqusition was recorded at ND filter = 25 % with an acqusition time of 20 s.