Supporting Information

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1 Supporting Information Capping Agent-Free Gold Nanostars Show Greatly Increased Versatility And Sensitivity For Biosensing Debrina Jana, Carlos Matti, Jie He, and Laura Sagle* Department of Chemistry, College of Arts and Sciences, University of Cincinnati, 301 West Clifton Court, Cincinnati OH *Corresponding author Tel: ; Fax: S1

2 Normalized Absorbance Nanostars With PVP Nanostars Without PVP Nanostars With PVP, Again Figure S1. Removal of PVP, followed by its addition once again, to demonstrate the recovery of the same LSPR frequency upon PVP coating. S2

3 Normalized Absorbance With PVP Without PVP With PEG-Biotin With Streptavidin Plasmon Shift (nm) With PEG-Biotin Without PEG-Biotin K d = 1 x M E-14 1E-13 1E-12 1E-11 1E-10 1E-9 1E-8 1E-7 1E-6 Streptavidin Concentration (M) λ shift (PVP removal) λ shift (biotin addition) λ shift (protein addition) -43 ± ± ± 2.5 Figure S2. Addition of streptavidin protein to borohydride treated gold nanostars with and without biotin. To ensure that the measured binding curve obtained for streptavidin binding to the treated nanostars is not due to non-specific binding, streptavidin was added to treated nanostar samples that did not contain the biotin-peg-thiol linker. LSPR shifts of 5-6 nm were observed at all the concentrations of streptavidin tested, indicating some non-specific binding is taking place. However, the LSPR shifts observed for samples containing the biotin linker are much larger and map out the expected sigmoidal binding curve. Thus, the non-zero values at the low end of the binding curve match these values measured for non-specific binding. Total shifts of 51 nm are observed, much larger than ~9 nm observed with untreated gold nanostars. In addition, the binding constant, K d, matches well with values observed for streptavidin binding to a surface. S3

4 1.0 w PVP w biotin w streptavidin 0.8 Absorbance 0.0 λshift (Biotin addition) λshift (Streptavidin addition) + sign indicates shifts with respect to w PVP Figure S3. Addition of streptavidin protein to untreated gold nanostars. As shown in the table, upon addition of streptavidin, LSPR shifts are observed. However, these shifts are 4-5 times smaller than those observed with borohydride treated gold nanostars. S4

5 Normalized Absorbance With PVP Without PVP With MUA+Thiol With Antibody With Antibody+PSA Plasmon Shift (nm) With Antibody+PSA Without Antibody+PSA K d = 9 x M E-12 1E-11 1E-10 1E-9 1E-8 1E-7 1E-6 PSA Concentration (M) λ shift (PVP removal) λ shift (thiol SAM addition) λ shift (antibody addition) λ shift (PSA addition) -41 ± ± ± ±2.8 Figure S4. Addition of PSA protein to borohydride treated gold nanostars with and without antibody. To ensure that the measured binding curve obtained for PSA binding to the treated nanostars is not due to non-specific binding, PSA was added to treated nanostar samples that did not contain the antibody. LSPR shifts of 7-8 nm were observed at all the concentrations of PSA tested, indicating some non-specific binding is taking place. However, the LSPR shifts observed for samples containing the PSA antibody are much larger and map out the expected sigmoidal binding curve. Thus, the non-zero values at the low end of the binding curve match these values measured for non-specific binding. Total shifts of 127 nm are observed, and the binding constant, K d, matches well with values observed for PSA binding in other assays. S5

6 0.7 Absorbance Figure S5. The noise of the instrument was determined by measuring the same sample of PVP-coated nanostars 4 times resulting in LSPR peak values of: , , , giving a standard deviation of 0.5 nm. S6

7 Absorbance without PVP with antibody with M PSA 1100 Plasmon shift (nm) with antibody+psa aggregate in buffer without antibody+psa aggregate in buffer K d = 9 x M 1E-20 1E-19 1E-18 1E-17 1E-16 1E-15 1E-14 PSA antigen concentration (M) λ shift (PVP removal) λshift (antibody addition) λ shift (PSA addition) -41 ± ± ± 3.5 Figure S6. PSA aggregation assay with borohydride treated gold nanostars with and without antibody. To ensure that the measured binding curve obtained for PSA-induced aggregation of the treated nanostars is not due to non-specific binding, PSA was added to treated nanostar samples that did not contain the antibody. LSPR shifts of 6-8 nm were observed at all the concentrations of PSA tested, indicating some non-specific binding is taking place. However, the LSPR shifts observed for samples containing the PSA antibody are much larger and map out the expected sigmoidal binding curve. Thus, the non-zero values at the low end of the binding curve match these to the measured for non-specific binding. Total shifts of 214 nm are observed, significantly larger than the shifts observed with PSA binding directly to the treated nanostars. S7

8 1.0 antibody bound Au NS antibody bound Au NS+ antibody-psa antigen mixture 0.8 Absorbance 0.0 Waveleng th (nm) Figure S7. Control Experiment #2. One batch of monoclonal PSA antibody (10-9 M) was mixed with 10-6 M PSA and allowed to bind for a few hours. Next, this mixture was added to gold nanostars containing PSA antibody. This mixture was compared to the addition of just 10-6 M PSA to antibody-coated nanostars and a shift of +1.7 nm was observed in the LSPR peak. S8

9 1.0 PSA antibody bound Au NS PSA antibody bound Au NS+ Bovine serum albumin 0.8 Absorbance 0.0 Figure S8. Control Experiment #3. When 1 mg/ml bovine serum albumin (BSA) solution was added to monoclonal antibody (10-9 M) attached gold nanostars, a shift of +8 nm was observed. S9

10 In order to gain an understanding of surface coverage by the protein molecules at saturated concentrations, 10-6 M, the following relation was utilized yielding the maximum plasmon shift due to the monolayer surface coverage of nanostars by a self-assembled monolayer. 1,2 R =m n n exp 1 exp, (1) Where m is the refractive index sensitivity or m-value (474 nm/riu), n SA and d SA are refractive index and thickness of the adsorbed analyte (streptavidin), n ext is the bulk refractive index of the external medium, which has been taken as 1.33 for water here, and d SAM and l d are the thickness of the biotin layer and field decay length of nanostars respectively. The index of refraction of the streptavidin layer, n SA, has been taken as whereas d SAM for streptavidin was considered as ,4 The electromagnetic field decay length, l d, has been shown to be sensitive to the shape of the nanoparticle and was found to be 7.5 nm for gold nanostars. 5,6 Solving the above equation for d SA,max and using d SA,max = Γ SA,max V SA where V SA, the molecular volume of streptavidin, is ( nm 3 ) and R max = 51 nm for streptavidin system, Γ SA,max value is calculated to be molecules/cm 2. S10

11 References (1) Haes, A. J.; Van Duyne, R. P. A nanoscale optical blosensor: Sensitivity and selectivity of an approach based on the localized surface plasmon resonance spectroscopy of triangular silver nanoparticles. J. Am. Chem. Soc. 2002, 124, (2) Jang, L.S.; Campbell, C. T.; Chinowsky, T. M.; Mar, M. N.; Yee, S. S. Quantitative interpretation of the response of surface plasmon resonance sensors to adsorbed films. Langmuir 1998, 14, (3) Schoch, R. L.; Lim, R. Y. H. Non-Interacting Molecules as Innate Structural Probes in Surface Plasmon Resonance. Langmuir 2013, 29, (4) Branch, D. W.; Wheeler, B. C.; Brewer, G. J.; Leckband, D. E. Long-term stability of grafted polyethylene glycol surfaces for use with microstamped substrates in neuronal cell culture. Biomaterials 2001, 22, (5) Haes, A. J.; Zou, S. L.; Schatz, G. C.; Van Duyne, R. P. A nanoscale optical biosensor: The long range distance dependence of the localized surface plasmon resonance of noble metal nanoparticles. J Phys Chem B 2004, 108, (6) Lee, J.; Hua, B.; Park, S.; Ha, M.; Lee, Y.; Fan, Z.; Ko, H. Tailoring surface plasmons of high-density gold nanostar assemblies on metal films for surface-enhanced Raman spectroscopy. Nanoscale 2014, 6, S11