Label-free SERS Selective Detection of Dopamine and Serotonin. Using Graphene-Au Nanopyramid Heterostructure

Transcription

1 Supporting nformation for Label-free Selective Detection of Dopamine and Serotonin Using Graphene-Au Nanopyramid Heterostructure Pu Wang 1, Ming Xia 1, Owen Liang 1, Ke Sun 1,2, Aaron F. Cipriano 3, Thomas Schroeder 4, Huinan Liu 3 and Ya-Hong Xie 1,* 1: Department of Materials Science and Engineering and California Nano Systems nstitute, University of California Los Angeles, Los Angeles, CA , USA. 2: WaferTech, LLC, 5509 NW Parker St., Camas, WA 98607, USA. 3: Department of Bioengineering, Materials Science and Engineering Program, University of California at Riverside, Riverside, CA 92521, USA. 4: nnovations for High Performance (HP), m Technologiepark 25, Frankfurt(Oder), D-15236, Germany. * Address correspondence to yhx@ucla.edu. 1

2 1. Stability of the Hybrid Platform The commonly used Ag nanoparticles (NPs) in suffer from serious problems of chemical reactivity. t has been reported that AgNPs will lose 50% of efficiency after 30 days. Moreover, the AgNPs cannot be reused and the cost makes it prohibitive for practical applications. Our gold tip substrate shows excellent chemical stability and can be easily recycled. t shows almost no efficiency lost at the time period of a year (Fig. S1). The lithographic nature of fabricating the pyramids offers the possibility of fabricating surfaces with pre-determined density and location of hot spots. n addition, the shape of the apex has a 4-fold symmetry in clear contrast to spherical nano-particles used in majority of the studies. Figure S-1. The average enhancement factor of Au tipped substrate with different time durations. 2. FDTD Simulation of the Au Tip substrate We modeled the enhancement of the electric field of Au nano-pyramids using Finite-difference time-domain (FDTD) analysis. The hexagonally arranged 200nm Au nano-pyramids geometry representative of the actual sample structure was used. Fig. S2a-c shows the x-z and x-y views of the electric field amplitude distribution for incident light wavelength 633nm. Strong field enhancement appears in between two individual tips as well as at the apex. However, the volume of the high field region in between the tips is approximately one order of magnitude larger compared to that at the apex and is expected to be the dominant contributor to the detected Raman signal. 2

3 Figure S-2. x-z and x-y views of the electric field amplitude distribution for incident light wavelength 633nm 3. Enhancement Factor (EF) of Dopamine and Serotonin Theoretically EF (ref. 1) is divided into electromagnetic (EM) and chemical (CM) components according to its mechanisms: EF= EF EF EM CM To describe the EM part of 4 E -approximation (ref. 2) is commonly used. For simplicity we can ignore the Raman shift and consider the EM part of EF as EF EM 0 L 4 ( ωl ) E E ( ω ) 4 where ω L and E 0 is frequency and electric field of the incident excitation laser, E is the local electric field. E is strongly affected by plasmonic response and is typically much higher than E 0. Chemical enhancement is usually considered as an analyte specific perturbation (the value of EF CM is being typically on the order of 1 ~ 10) to the EM part of EF. The E approximation for enhancement is an simplification, but provides a very useful yardstick estimate for experimental enhancement in a single molecule located at x 0. n our work, the ratio E/E 0 is ~100 at hot spots per FDTD (finite difference time domain) calculation, which is in line with most reported results in the literature (ref. 3). The EF EM ~10 8 we obtained experimentally agrees with the FDTD result and is also in line with the values 3

4 reported by others. CM based EF is dependent on the molecule - surface interaction. Graphene EF The EF of graphene can be determined by comparing graphene characteristic band intensities from (Au tip) and (flat Au film) spectra. n the previous work of ours (ref. 4), we have estimated the average EF for Au tip substrate as ~10 8. Molecule EF n the practical experiments, EF is defined as the ratio of signal intensity to the Raman signal intensity that is obtained for exactly the same molecule in the absence of the substrate, with all other conditions identical (ref. 3). To this end we have EF = / c / c where is the intensity of analyte in solution with known concentration of c, is the Raman intensity of analyte in solution with much higher concentration c under the same conditions (the same Raman spectroscopic setup). This is also known as analytical EF. One disadvantage of the analytical EF originates from the volume concentration of analyte c since is a surface process. To remedy this problem the substrate EF is introduced as EF = / N / N Surf Vol where N Vol is the number of analyte molecules contributing to bulk Raman signal, N Surf is the number of absorbed analyte molecules contributing to Raman signal. The probe molecules are assumed to disperse uniformly on the substrates. The specimens for and are prepared and detected in the same conditions. The number of molecules under detection can be estimated as 4

5 N= S laser NAMV S sub Where N is Avogadro constant, M is the molar concentration, V is the volume of solution droplet, S is the size of the substrate and S is the size of the laser spot. Figure S-3. (10-10 M) and Raman scattering (10-2 M) of dopamine Figure S-4. (10-10 M) and Raman scattering (10-2 M) of serotonin The enhancement factor (EF) of the graphene - Au tip substrate is measured by diluting molecular solution of M with 633nm laser excitation. A droplet of ~2µL solution is deposited on the hybrid system and it diffuses on the surface to form a blot with diameter of ~2mm. As reference, a 2µL droplet of 0.01 M molecular solution is deposited on graphene/flat Au film substrate for Raman scattering () measurement. The and non- spectra 5

6 of dopamine and serotonin are presented in Fig. S3 and S4 separately. Considering the 1482 cm -1 peak assigned to dopamine, we obtained EF of ~2*10 8 for dopamine on the hybrid platform. For serotonin, the EF is ~10 9 M. 4. Molecule Peak ntensity vs Concentration Figure S-5. Dopamine 1482 cm -1 peak intensity vs concentration Figure S-6. Serotonin 1547 cm -1 peak intensity vs concentration Table S-1. Vibration mode dependent enhancement and assignment of Raman peaks in spectra for dopamine (ref. 5) 6

7 Peaks (cm -1 ) Tentative assignment 591 C-H rocking 635 Phenol O-H op bending 771 Ring breathing 816 Aromatic C-H def op 1267 phenolic C-O stretching 1358 C-H ip bending 1424 ip CH 2 scissoring 1482 phenyl C=C stretching Table S-2. Vibration mode dependent enhancement and assignment of Raman peaks in spectra for serotonin (ref. 6,7) Peaks (cm -1 ) Tentative assignment 756 ndole ring breathing 936 O-H op deformation; C-H op deformation 1008 Benzene ring breathing 1185 C-H ip deformation 1253 C-C and C-N stretching in pyrrole 1350 C-N stretching, indole ring vibration 1423 C=C, C=N ip vibration in pyrrole 1448 Pyrrole ν(n C=C), benzene δ(ch) 1550 pyrrole ring stretching ν(c=c) 7

8 1620 C=C stretching * The ν(x-y) is a vibration mode assigned to an X-Y bond stretching vibration, δ denotes deformation coordinate, ip denotes in-plane and op denotes out-of-plane. References: (1) Le Ru, E. C.; Etchegoin, P. G. M bulletin 2013, 38, (2) Le Ru, E.; Etchegoin, P. Chemical Physics Letters 2006, 423, (3) Le Ru, E.; Blackie, E.; Meyer, M.; Etchegoin, P. G. The Journal of Physical Chemistry C 2007, 111, (4) Wang, P.; Zhang, W.; Liang, O.; Pantoja, M.; Katzer, J.; Schroeder, T.; Xie, Y.-H. ACS nano 2012, 6, (5) Lagutschenkov, A.; Langer, J.; Berden, G.; Oomens, J.; Dopfer, O. Physical Chemistry Chemical Physics 2011, 13, (6) Perna, G.; Lasalvia, M.; Gallo, C.; Quartucci, G.; Capozzi, V. The Open Surface Science Journal 2013, 5, 1-8. (7) Razmutė-Razmė,.; Kuodis, Z.; Eicher-Lorka, O.; Niaura, G. Chemija 2007, 18,