Supporting information
For the synthesis of silver nanoparticles an experimental series with different conditions were performed for
optimization. The details are given in Table 1. In the trials, the effects of the molar ratio between the polymer
and AgNO3, type of reducing agent (L-ascorbic acid and NaBH4), pH of the reaction medium, addition speed of
silver precursor, addition sequence of reducing agent/AgNO3 and the amount of reducing agent on the UV
absorption of silver nanoparticles were investigated. As it is known the silver nanoparticles give a strong
absorption in the lower range of UV visible spectrum which is usually called a surface plasmon resonance
absorption peak (SPR). This absorption peak gives information about the diameter, shape and size distribution
of the nanoparticles. Smaller sized and spherical nanoparticles usually give a maximum absorption at around
380-410 nm range where a shifting to higher wavelengths usually refers to bigger particles and/or change into
other shapes (triangles, wire, etc.) rather than spherical. The breadth of the absorption peak is also important
and related to the size distribution of the nanoparticle. Broader peaks usually refer to more polydisperse
nanoparticle sizes. Fig. 1 shows the comparison of UV absorption spectra of the silver nanoparticles
synthesized under different reaction conditions.
Table 1. Experimental series on silver nanoparticle synthesis
Trials
Molar Ratio
(PAAmSH/AgNO3)
Reducing
agent (RA)
Reactio
n pH
Addition
speed of
AgNO3
A
1/20
L-Ascorbic acid
10.5
fast
B
1/15
L-Ascorbic acid
10.5
fast
C
1/10
L-Ascorbic acid
10.5
fast
D
1/5
L-Ascorbic acid
10.5
fast
E
1/10
NaBH4
10.5
fast
F
1/10
L-Ascorbic acid
8.5
fast
G
1/10
L-Ascorbic acid
9.5
fast
H
1/10
L-Ascorbic acid
10.5
Slow
I
1/10
L-Ascorbic acid
10.5
fast
J
1/10
L-Ascorbic acid
10.5
fast
1
Addition
Sequence
L-Asc. Ac
/AgNO3
L-Asc. Ac
/AgNO3
L-Asc. Ac
/AgNO3
L-Asc. Ac
/AgNO3
L-Asc. Ac
/AgNO3
L-Asc. Ac
/AgNO3
L-Asc. Ac
/AgNO3
L-Asc. Ac
/AgNO3
AgNO3 /LAsc. Ac
L-Asc. Ac
Molar ratio of RA
/
PAAm-SH+AgNO3
1
1
1
1
1
1
1
1
1
0.65
K
1/10
L-Ascorbic acid
10.5
fast
/AgNO3
L-Asc. Ac
/AgNO3
2
The effect of the molar ratio of PAAm-SH to AgNO3 (Trials A-D) can be seen in Fig. 1a. There was no significant
difference on the UV absorbance of nanoparticles for the molar ratios of 1/10 and 1/5 where optimal particle
coverage was possibly achieved. However, when the amount of stabilizing polymer was decreased (1/15 and
1/20) the SPR peak of silver nanoparticles (~401 nm) shifted to higher wavelengths (419 and 427 nm,
respectively) which refers to bigger particle sizes. When the reducing agent was replaced with NaBH4 (Trial E Fig 1b) there was no shifting in SPR peak, however, the absorption was much broader indicating an increased
polydispersity. This can be due to the very fast reaction ability of NaBH4 that decreases the control on particle
nucleation leading to broad particle size distributions. In Fig. 2, a comparison of the particle size distribution of
the trials C and E is given. The particle size of C was much more homogenous than that of the nanoparticles
prepared with NaBH4.
The effect of the pH on the size of silver nanoparticles prepared via L-ascorbic acid was studied by Qin et al.
(2010) mentioning that higher pH values led to smaller particle sizes. Indeed, when the pH was lower (8.5) the
nanoparticles had a SPR peak at 421 nm and the absorption was broad with a shoulder between 500-550 nm
showing a red shifting possibly due to formation of different particle shapes (Fig 1c). With the increase of pH
the SPR peak shifted to lower values with a decrease in breadth of the absorption curve. The possible
explanation to this is that the number of the nuclei increased with increased pH due to the promoted reactivity
of the reducing ascorbic acid. In the trials the addition speed of AgNO3 to the reaction medium was also
evaluated (Trial H) and the result is given in Fig 1d. Interestingly when the silver was added in a slowly drop
wise the SPR peak of the silver nanoparticles shifted to a slightly higher value and a secondary absorption as a
shoulder at around 500 nm was observed. This is difficult to explain, however, it can be due to the difference of
the ratio of stabilizing polymer to silver precursor during the addition period as well as variations in particle
nucleation. In another trial the effect of the addition sequence of silver and reducing agent was examined (Trial
I). In respect to the usual method of later addition of silver into reaction medium including the reducing agent,
AgNO3 was first introduced into the reaction medium and ascorbic acid was subsequently added fast into the
system. In the latter case the pH of both reaction medium and ascorbic acid solution was adjusted to result in
final pH of 10.5. The later addition of ascorbic acid induced a small shift in the SPR peak and the spectrum was
slightly broader in comparison to the former addition style (Fig. 1e) which possible could be because of
relatively decreased speed of particle nucleation. Finally, the effect of the amount of reducing agent was also
2
checked in the trials (Trials J,K). The results showed that for the lower amounts of ascorbic acid (molar ratio
0.65 and 1) the absorption spectra were identical, however, further increase in ascorbic acid ratio (2) resulted
in a broader absorption in the spectrum (Fig 1f).
Figure 1. UV absorption spectra of the silver nanoparticles synthesized under different conditions; a) effect of
molar ratio PAAm-SH/AgNO3, b) effect of different reducing agents, c )effect of medium pH, d) effect of the
addition speed of AgNO3, e) effect of the addition sequence, f) effect of the molar ratio of reducing agent
3
Figure 2. Particle size distribution of the silver nanoparticles obtained from the trials C (L-Ascorbic acid) and E
(NaBH4)
References
Yaqiong Qin, Xiaohui Ji, Jing Jing, Hong Liu, Hongli Wu, Wensheng Yang, Size control over spherical silver
nanoparticles by ascorbic acid reduction, Colloids and Surfaces A: Physicochem. Eng. Aspects 372 (2010) 172–
176.
4