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Electronic Supplementary Material
Gold nanoparticles-based colorimetric determination of cationic surfactants in
environmental water samples via both electrostatic and hydrophobic interactions
Kamlesh Shrivas1*, Swapan Sahu1, Anupam Ghorai1 and Ravi Shankar2,3
1
Department of Chemistry, Guru Ghasidas Vishwavidyalaya, Bilaspur, CG-495009, India
2
Nanoscience and Nanoengineering Program, South Dakota School of Mines and
Technology, Rapid City, South Dakota-57701, USA
3
Fujifilm Imaging Colorants, Inc. 233 Cherry Lane, New Castle, Delaware-19720, USA
*Corresponding author: Email-kshrivas@gmail.com, Phone: +91-7752-260488
Calculation for molar concentration of AuNPs:
It was possible to estimate the concentration of NPs from TEM experiments. It can be assumed
(R. L. Johnston, 'Atomic and Molecular Clusters', Taylor & Francis, 2002) for “spherical” metal
clusters such as a gold nanoparticle that:
1. Determine the average number of atoms per nanoparticle using the following formula:
N = number of atoms per nanoparticle
π = 3.14159…
ρ = density of face centered cubic (fcc) gold = 19.3g/cm3 convert to g/nm3)
D = average diameter of nanoparticles from TEM measurement (D =4.0)
M = atomic mass of gold (197g/mol)
NA = number of atoms per mole (6.022x1023)
N=1.97x103 gold atoms per nanoparticle
2. Calculation of the amount of nanoparticles formed (NPs) when 50 mL of 0.25 x10-3 M (4.25
mg) solution of HAuCl4 was mixed with 0.25 mL of 0.1 M sodium tartrate/malonate and stirred
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for 20 min followed by the addition of 0.5 mL of 0.1 M NaBH4 to the solution mixture and
stirred for 20 min. So, the concentration (M) of AuNPs formed in this reaction. The calculation is
based on the given reference [1,2].
W= VxMxWt/1000
[Where W is the weight of the compound, V is the volume of solution, M is the molar
concentration and Wt is the molecular weight of the compound]
As law of mass action:
339.79 g weight of AuHCl4 will give a 196.97 g of Au
Then, 4.25x10-3 g of AuHCl4 will give a 2.46x10-3g of Au
Molar concentration of gold atoms
= 2.46x10-4 g mole gold atoms/L
No. of gold atoms, Natom = 2.46x10-4 x 6.022x1023 = 1.48x1020
3. Nanoparticles formed per mole:
NT = Natom /N =1.48x1020/1.97x103
NT=7.5x1016 nanoparticles formed per mole
4. Molar concentration of the tartrate capped gold nanoparticles in the solution is estimated by
dividing NT by Avagadro’s number (NA):
= 7.5x1016/6.022x1023
=125x10-9
Molar concentration= 125 nM
Similarly, for malonate capped AuNPs
N=1.32x103 gold atoms per nanoparticle
Natom= 1.48x1020 No. of gold atoms
NT= 1.12x1017 nanoparticles formed per mole
Molar concentration =185 nM
Similarly, for citrate capped AuNPs
N=3.85x103 gold atoms per nanoparticle
Natom= 1.48x1020 No. of gold atoms
NT= 3.84x1016 nanoparticles formed per mole
Molar concentration = 64 nM
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DLS measurements of citrate and maolnate capped AuNPs and after addition of CPC
(a)
Average size =5.5+1.2 nm
(b)
Average size =128+13 nm
(c)
Average size =3.5+0.8 nm
(d)
Average size =74+8 nm
Fig. S1. DLS measurement of citrate and malonate capped AuNPs before and after the addition
of CPC: (a) citrate capped AuNPs, (b) citrate capped AuNPs+CPC (c) malonate capped AuNPs,
and (d) malonate capped AuNPs+CPC
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Effect of surface functionalization of AuNPs for the detection of CS+
(a)
(b)
(c)
(d)
(e)
(f)
(a) AuNPs/Cit
(b) AuNPs/Cit+CPC
(c) AuNPs/Mal
(d) AuNPs/Mal+CPC
(e) AuNPs/Tar
(f) AuNPs/Tar+CPC
Fig. S2. Glass vial containing AuNPs capped with citrate (Cit), malonate (Mal), Tartrate (Tar)
groups and separate addition of CPC along with their respective UV-visible spectra: (a)
AuNPs/Cit, (b) AuNPs/Cit+CPC, (c) AuNPs/Mal, (d) AuNPs/Mal+CPC, (e) AuNPs/Tar and (f)
AuNPs/Tar+CPC
Optimization of the method for colorimetric determination of CS+ using AgNPs
The analytical variables such as pH, reaction time and concentration of AuNPs that affected the
detection of CS+ using AuNPs as a colorimetric probe were optimized. In the present
investigation, CPC was selected as a model compound for optimization of analytical
performance and determination of CS+ from water sample. The absorbance ratio obtained at 740
nm and 540 nm was directly related to the degree of aggregation of NPs. This absorbance ratio of
the solution mixture was used to evaluate the effect of analytical variables for the determination
of CS+.
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The influence of pH on the aggregation/absorbance ratio NPs was observed for the detection
of different types of target analytes from the sample solution. The citrate coated AuNPs is found
stable at pH higher than 6.0 (pH>pI of citrate). The carboxylic group of citrate coated NPs have a
negative charge on the surface of NPs that causes the repulsion of each NPs to prevent from the
aggregation. However, the addition of analytes causes the aggregation of NPs due to the
exclusion of repulsive forces between the NPs [3-5]. Therefore, effect of different pH (3.0 to
11.0) on the aggregation/absorbance ratio of NPs solution was studied using tartrate capped
AuNPs for the detection of CPC (100gmL-1), shown in Fig. S3(a). The absorbance ratio of
solution mixture was increased with increasing the pH of solution from 3.0 to 9.0 and after there
was no change in the absorbance ratio was obtained. Hence, the pH of 9.0 was chosen for
remaining experiments for the colorimetric detection of CS+ from the sample solution.
Reaction time is an important parameter that affects the aggregation/absorbance ratio of NPs
in the solution mixture, because increase in time will give an enough time for surface interactions
of analyte molecules to AuNPs. Hence, different reaction times such as 1, 2, 3, 4, 5, 6 and 7 min
were tested for the detection of CS+ by spiking a CPC (100 ngmL-1) in six different glass vial
containing AuNPs with pH 9.0 at given time intervals. The results are shown in Fig. S3(b). The
aggregation and absorbance ratio of solution mixture was increased with increasing the reaction
time from 1 to 5 min and after there was no any change was observed up to 7 min. Thus, a
reaction time of 5 min was found best and selected for the subsequent experiments.
Concentration of NPs also affects the aggregation/absorbance ratio of NPs solution mixture
because increasing the concentration of NPs will decrease the inter-particle distance and cause
the increase in aggregation. Thus, different concentrations of NPs in the range of 2.5-30 nM were
tested for the colorimetric detection of CS+ at the optimized conditions of the method, shown in
Fig S3(c). The absorbance ratio of solution was linearly increased from 2.5 to 25 nM
concentration of AuNPs and after there was decrease in the absorbance ratio. The use of higher
concentration of AuNPs (>25 nM) was not good because of a rapid flocculation of NPs followed
by the settling on the bottom of the glass vial. Therefore, 25 nM of AuNPs was sufficient to get
the optimum aggregation and detection of CS+ from water sample.
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Fig. S3. Effect (a) pH, (b) reaction time and (c) concentration tartrate capped AuNPs on the
aggregation/absorbance ratio for the detection of CS+ in water samples
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(a)
(b)
(c)
(d)
(e)
(a) AuNPs
(b) AuNPs+ Na+
(c) AuNPs+ Cu2+
(d) AuNPs+ TX-100
(e) AuNPs+ CPC
Fig. S4. Glass vial containing (a) tartrate capped AuNPs, (b) tartrate capped AuNPs with Na+, (c)
tartrate capped AuNPs with Cu2+, (d) tartrate capped AuNPs with TX-100 and (e) tartrate capped
AuNPs with CPC and their respective UV-visible spectra
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