Supplemental Info for:
UV-induced photochemical transformations of citrate capped silver nanoparticle suspensions
Justin M. Gorham, Robert I. MacCuspie, Kate L. Klein, D. Howard Fairbrother, R. David
Holbrook
Silver nanoparticle (AgNP) synthesis
AgNPs were prepared using established protocols (see main text for citations).
Specifically, 1.690 mL of 58.8 mmol•L
-1
aqueous silver nitrate and 2.920 mL of 34 mmol•L
-1
aqueous tribasic sodium citrate dehydrate were added to 400 mL of boiling deionized (DI) water while stirring rapidly, before drop-wise addition of 2.000 mL of 100 mmol•L
-1
aqueous sodium borohydride (NaBH
4
). The reaction was stirred for 30 min at a slow boil, then allowed to cool to room temperature on the hot plate. The cooled AgNP product solutions were purified and concentrated by pressurized stirred cell ultrafiltration using 10 kDa relative molecular weight cut-off regenerated cellulose filters (Millipore,
Billerica, MA). This removed any unreacted reagents into the filtrate while the AgNPs remained in the retentate. Before experiments, the retained AgNP solutions were passed through a 0.2 µm polyvinylidene fluoride (PVDF) syringe filter to remove adventitious dust or large aggregates. Ultrafiltration and purification typically increased the total silver concentration to approximately 0.4 mg
 mL -1 .
Synthesis of “In-House” silver nanoparticle (AgNP) suspensions and diluents for
AFM and DLS measurements were prepared using DI water with a resistivity


18.2 M
 •cm obtained from Aqua Solutions (Jasper, GA, USA) Type I biological grade water purification system, outfitted with an ultraviolet lamp to oxidize residual organics and a low relative molecular weight cut-off membrane.
Donnan Membrane (DM) Technique Description
The DM technique, originally described by Temminghoff (Temminghoff et al.
2000), was utilized to quantify Ag
+
concentrations in the presence of AgNPs. This was of particular importance due to the presence of residual oxidized AgNPs in suspension which did not demonstrate any detectable absorbance. As depicted schematically in
Figure S1 A), the DM technique operates with a donor volume and an acceptor volume, both of which are semi-sealed in containers to prevent evaporative effects. The donor volume (2000 mL) contained the exposed/unexposed AgNP suspension or silver nitrate
(at a silver concentration of 0.1 mg•L -1 ) in a buffer consisting of 0.1 mmol•L -1 Ca(NO
3
)
2
,
0.07 mmol•L
-1
KNO
3
and 0.014 mmol•L
-1
NaNO
3
. The acceptor volume (approximately
35 mL) contained only the aforementioned buffer plus 0.005 mmol•L -1 of trisodium citrate to account for the sodium cations in the AgNP suspensions. Both solutions were pumped via a peristaltic pump into the DM cell at a rate of 2.5 mL•min
-1
.
The cell (Figure S1 B)) consisted of two compartments, one for the donor and one for the acceptor solutions, which were separated by a cation exchange membrane. The cation exchange membrane is designed with pores that permit cations to diffuse through the membrane while preventing and transfer of particulate material. Any Ag
+ concentration in the donor, therefore, will follow Le Chatlier’s principle and exchange towards equilibrium on both sides of the membrane. Since the donor volume was

relatively large compared to the acceptor, it is estimated that the nanoparticle-to-ionic equilibrium was affected by < two percent during the Ag + transfer from the donor to the acceptor. At specified intervals, 2.0 mL samples were removed for ICP-MS analysis from both the donor and the acceptor. The relative percentage of Ag
+
in the donor was estimated by determining the ratio of acceptor:donor. The equilibrium concentration was determined by fitting an acceptor:donor versus time plot with an exponential rise to maximum equation.
Cleaning and Solution conditions
Prior to irradiation, all quartz vials were cleaned in a 1 mol

L
-1
nitric acid solution for over 4 h. Nitric acid (≈ 70 % v/v) and ethanol (> 95 %) were obtained from J. T.
Baker (Phillipsburg, NJ) and Warner Graham Company (Cockeysville, MD), respectively. The vials were further washed with purified water and ethanol and allowed to dry overnight prior to use. Complete drying was found to be the key to experimental success as even traces of residual ethanol resulted in no observable reaction. XPS silicon substrates were pre-cleaned with ethanol and lint-free wipes after being cut to the appropriate size.
Solution pH was monitored throughout irradiation of the 20 nm and 80 nm
AgNPs (Nanocomposix) in the kinetic studies. 20 nm AgNPs varied minimally and maintained a steady pH of 7.81 ± 0.05 (average and standard deviation of 18 measurements). The 80 nm AgNPs exhibited a slight pH increase from 7.65 to 7.85 pH units in early exposures, and then gradually returned to the initial pH at long irradiation times; the average pH was 7.77 ±0.08 (average and standard deviation of 16

measurements) over the duration of the experiment. Based on the manufacturer’s specifications, the sodium concentration in the AgNP suspensions was 0.8 mM after dilution to 4.00 mg

L
-1
and 1.3 mM after dilution to 6.61 mg

L
-1
(select 20 nm samples only).

Figure S1: A.
Schematic representation of the setup for the Donnan
Membrane Technique (DMT). A large volume (2000 mL) of the test
AgNP suspension diluted into an appropriate buffer was contained in the
( 1.
) donor while a relatively small volume (35 mL) of only buffer made up the ( 2.
) acceptor solution. Both solutions were circulated by a ( 3.
) peristaltic pump at 2.5 ml/min and into the ( 4.
) DMT cell where the cation exchange occurred. B.
The flow through cell consisted of two parts, the ( 5.
) donor side and ( 6.
) the acceptor side separated by a ( 7.
) cation exchange membrane. The ion(s) of interest (in this case, Ag
+
) will transfer from the donor solution, through the cation exchange membrane
( 7.
), and into the acceptor solution (Ca
2+
is the counter ion from the membrane). The advantage of this technique is that only ionic species are transferred into the acceptor solution; particulate and nanoparticles are retained in the donor solution.

Figure S2: UV exposed (circles) and shielded (squares) 20 nm
AgNPs (Nanocomposix, 6.6 mg

L
-1
) confirm that the SPR decrease was photo-induced. Lines are guides to the eye.

2
1
4
3
6
5
0
0 10 20 30
Irradiance (W * m
-2
)
40 50
Figure S3: Variation in k obs
for 20 nm AgNPs
(Nanocomposix, 4 and 6.6 mg

L
-1
) exposed for a constant light exposure at different irradiance. The solid line is a fit to the relationship, k obs
= a ∙ (Irradiance) n . The non-linear trend
(n ≈ 7) indicates a multiphoton process.

Figure S4: Intensity-based size distributions as measured by DLS for unexposed
(top row) and irradiated (t exp
≈ 8000 min) (bottom row) of differently sized AgNP suspensions. Error bars and each data point are representative of one standard deviation of the average of at least 5 separate measurements.

Figure S5: Absorbance Spectra of in-house synthesized AgNP suspensions prepared for liquid TEM analysis. Suspensions were 4.0 mg•L -1 of AgNPs and 49.2 mg•L -1 of citrate.
Suspensions were exposed for approximately
5300 minutes of UV radiation.

References
Temminghoff, E. J. M., A. C. C. Plette, et al. (2000). "Determination of the chemical speciation of trace metals in aqueous systems by the Wageningen Donnan
Membrane Technique." Analytica Chimica Acta 417 (2): 149-157.