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Sensitive and selective localized surface plasmon resonance light-scatteringsensor for Ag+ with unmodified gold nanoparticles

Chengke Wu, Cen Xiong, Linjing Wang, Chongchong Lan and Liansheng Ling*

Received 3rd April 2010, Accepted 8th July 2010

DOI: 10.1039/c0an00201a

A novel localized surface plasmon resonance (LSPR) light-scattering sensor for Ag+ was developed

with unmodified gold nanoparticles (AuNPs), based upon the specific recognition property of Ag+ with

a cytosine–cytosine mismatched base pair. The addition of Ag+ induced the oligonucleotide 50-TAC

ATA CAT ACT ATC TAT CTA-30 to be desorbed from the surface of the AuNPs, resulting in the

aggregation of AuNPs, accompanied by a dramatic enhancement of the LSPR light-scattering

intensity. The enhancement of LSPR light-scattering intensity was proportional to the concentration of

Ag+ in the range of 0.13–1.12 mM, with a limit of detection of 62.0 nM. The results were also proved by

a colorimetric method. Furthermore, this method can provide satisfactory results for the determination

of Ag+ in water samples and industrial products.

Introduction

Silver ions play a significant role in the photography, pharmacy

and imaging industries. With the increase in the application of

Ag+, much more attention has been paid to its potential toxicity.1

Silver ions exist in water in the form of inorganic or organic

compounds, which could be bioaccumulated by aquatic

creatures.2,3 The accumulation of Ag+ can restrain the activity of

protein, influence the reproduction of aquatic creatures and

terrestrial organisms. Thus, a rapid and sensitive analysis of Ag+

in environmental or industrial samples is of vital importance. For

these reasons, several methods for Ag+ analysis have been

developed, such as fluorescence spectrometry,4,5 neutron activa-

tion analysis,6 electrochemistry,7–9 atomic absorption spectrom-

etry,10,11 inductively coupled plasma mass spectrometry,12

spectrophotometry13,14 and scanning electrochemical micros-

copy.15 Some of these can achieve a detection limit at the ppm

level.

Au nanoparticles (AuNPs) have specific photonic properties

and exhibit various surface plasmon resonance and surface

plasmon absorption with different sizes. The color of AuNPs is

red when they are dispersed in water, but it changes into purple,

even blue, when aggregation of the AuNPs occurs.16–20 The

assembly and disassembly of AuNPs can be controlled by DNA

hybridization and other recognition processes. Colorimetric

sensors such as those involving the detection of DNA,16,19–22

RNA,23 protein,24–26 small molecules,27,28 and metal ions29–38 have

been reported based upon each recognition process. Recently,

a sensitive and selective colorimetric sensor for Ag+ was devel-

oped by Li et al.39 and Zhou et al.40 using G-quadruplex DNA.

The detection limit of most of these assays was in the mM range,

only a few reached the nM range.

Localized surface plasmon resonance (LSPR) is a photon-

driven coherent oscillation of the surface conduction electrons,

which are excited by electromagnetic radiation. The light

School of Chemistry and Chemical Engineering, Sun Yat-Sen University,Guangzhou, 510275, People’s Republic of China. E-mail: cesllsh@mail.sysu.edu.cn.; Tel: 0086-20-84110156

2682 | Analyst, 2010, 135, 2682–2687

interacts with particles much smaller than the incident wave-

length and the electrons in the nanoparticles oscillate locally

around the nanoparticles. Moreover, LSPR light-scattering

happens when the electromagnetic wave frequency is the same as

that of the oscillating electron.41 Because of its good sensitivity

and convenient apparatus manipulation, the LSPR light-scat-

tering method based on AuNPs has been used for the determi-

nation of amino acids,42,43 biomolecules44–46 and other samples.47

Here we report a new Ag+ sensor with unmodified AuNPs based

upon its specific C–C mismatch recognition property, which

demonstrates a new strategy to investigate other types of metal–

DNA interaction in the future.

Experimental

Chemicals and materials

HAuCl4$3H2O was purchased from Sigma-Aldrich Co., and

oligonucleotides were synthesized by Shanghai Sangon Biolog-

ical Engineering Technology & Services Co., Ltd. (Shanghai,

China). All reagents were AR unless mentioned otherwise, and

the water was double distilled water.

Design of the oligonucleotide

Oligodeoxyribonucleotide with the sequence of 50-TAC ATA

CAT ACT ATC TAT CTA-30 (oligo-1) was designed for our

research. As shown in Fig. 1, it can adsorb on the surface of

AuNPs and prevent the aggregation of AuNPs in the presence

of a sodium salt. The addition of Ag+ leads to the formation of

dsDNA and desorption of the oligonucleotide from the gold

nanoparticles. Oligodeoxyribonucleotide 50-ACA ACA ACA

ACA TCT TTT TTT-30 (oligo-2), which contains the same base

contents but a different sequence to that of oligo-1, was selected

as a control sequence.

Instruments

Intensity and LSPR light-scattering spectra were measured with

an RF-5301PC spectrofluorimeter (Shimadzu, Japan) with an IR

This journal is ª The Royal Society of Chemistry 2010

Fig. 1 Scheme of recognizing a Ag+ ion with unmodified AuNPs, based

upon its specific binding to a C–C mismatched base pair.

Fig. 2 LSPR light-scattering spectra of AuNPs–oligonucleotide with

and without Ag+. a) 0.78 nM AuNPs + 0.5 mM oligo-1 + 0.02 M NaNO3;

b) a + 0.5 mM Ag+; c) 0.78 nM AuNPs + 0.5 mM oligo-2 + 0.02 M

NaNO3; d) c + 0.5 mM Ag+.

Fig. 3 Solution color and absorption spectra of the AuNPs–oligonu-

cleotide with and without Ag+. a) 3.87 nM AuNPs + 3.23 mM oligo-1 +

0.096 M NaNO3; b) a + 6.45 mM Ag+; c) 3.87 nM AuNPs + 3.23 mM

oligo-2 + 0.096 M NaNO3; d) c + 6.45 mM Ag+.

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sensitive detector. All absorbance measurements were made with

a TU-1901 double-beam spectrophotometer (Beijing Purkingje

General Instrument Co., Ltd., China). Transmission electron

microscopy (TEM) images were recorded with a JEM-2010HR

transmission electron microscope (JEOL Ltd., Japan).

Preparation of Au nanoparticles

AuNPs with an average diameter of 13 nm were prepared using

the method of the reaction of HAuCl4 with citrate. Sodium

citrate solution (3.0 mL, 38.8 mM) was added to a stirred boiling

solution of HAuCl4 (30 mL, 1.0 mM), after the color turned to

red, keeping the solution boiling for another 20 min, then it was

cooled down to room temperature, and the AuNPs were

collected after filtering through a 0.45 mm filter membrane.

Preparation of samples for LSPR light-scattering detection

Oligo-1 (100 mM, 10 mL) with different concentrations of Ag+

ions was added to the gold nanoparticles (10.44 nM, 150 mL),

then 150 mL buffer (PBS, 10 mM) was injected into the solution

to control the pH value, and 15 mL NaNO3 (2.0 M) was added to

control the ion strength. Finally, the mixture was diluted to

2.0 mL with distilled water for LSPR light-scattering determi-

nation. All of the LSPR light-scattering spectra were obtained by

simultaneously scanning the excitation and the emission wave-

length (namely Dl ¼ 0 nm) from 300 to 800 nm, by keeping the

slit width for excitation and emission at 3.0 nm.

Preparation of samples for colorimetric detection

Oligo-1 (100 mM, 10 mL) with a quantity of Ag+ (100 mM, 20 mL)

was added to the AuNPs (10.44 nM, 115 mL), then 150 mL buffer

solution (PBS, 10 mM) was injected into the solution to control

the pH, and 15 mL NaNO3 (2.0 M) was added subsequently.

Results and discussions

Spectra characteristics

The LSPR light-scattering spectra of AuNPs–oligonucleotides

with and without Ag+ were studied. As demonstrated in Fig. 2,

the LSPR light-scattering spectrum of AuNPs–oligo-1 changed

noticeably after the addition of Ag+. When AuNPs–oligo-1 was

observed in the absence of Ag+ ions, the LSPR light-scattering

spectrum was weak within the range of 300–800 nm (curve a),

This journal is ª The Royal Society of Chemistry 2010

while the intensity was enhanced dramatically after the addition

of Ag+ ions, and the light scattering intensity reached a maximum

peak at 665 nm (curve b). This phenomenon suggests the

aggregation of AuNPs.41 Comparatively, the AuNPs with

a control sequence of oligo-2 showed no change in the LSPR

light-scattering spectrum before and after the addition of Ag+

(curve c, d).

The effect of Ag+ on the solution color and the absorption

spectrum was investigated as well. It is demonstrated in Fig. 3

that the solution of AuNPs–oligo-1 was red with a maximum

absorption peak at 520 nm in the presence of 0.096 M NaNO3,

while it changed from red to purple after Ag+ was added,

accompanied by a dramatic decrease in absorption intensity at

520 nm and the appearance of a new absorption band at a longer

wavelength. These phenomena may be due to the aggregation of

AuNPs,19 which is in good accordance with the results of the

LSPR light-scattering spectra. However, the addition of Ag+ had

little effect on the solution color and the absorption spectrum of

AuNPs–oligo-2, which revealed that the Ag+ ions could not

induce the aggregation of AuNPs with the oligo-2.

To elucidate the phenomena of the Ag+-triggered LSPR light-

scattering spectra enhancement, the dispersion and aggregated

states of AuNPs induced by silver ions were studied using TEM

images. As displayed in Fig. 4, only a little aggregation of AuNPs

existed in AuNPs–oligo-1 (image a) and AuNPs–oligo-2

(image c) in the presence of NaNO3. However, when Ag+ was

added, a manifest aggregation was observed for AuNPs–oligo-1

Analyst, 2010, 135, 2682–2687 | 2683

Fig. 4 TEM images of the AuNPs–oligo-1 and AuNPs–oligo-2 with and

without Ag+. Experimental conditions: a) 0.78 nM AuNPs + 0.5 mM

oligo-1 + 0.02 M NaNO3; b) a + 0.5 mM Ag+; c) 0.78 nM AuNPs +

0.5 mM oligo-2 + 0.02 M NaNO3; d) c + 0.5 mM Ag+.

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(image b), while there was almost no change for AuNPs–oligo-2

(image d). This difference between AuNPs–oligo-1 and AuNPs–

oligo-2 induced by Ag+ was in quite good accordance with the

results of the LSPR light-scattering spectra, absorption spectra

and color change.

The addition of Ag+ induced different changes for AuNPs–

oligo-1 and AuNPs–oligo-2, when examined separately, which

might result from the different interaction between the oligonu-

cleotide and Ag+. The melting temperature (Tm) was used to

interpret the virtue of the interaction between the oligonucleotide

and Ag+. As shown in Fig. 5, the melting temperature of oligo-1

was about 21 �C, and it increased to 33 �C with the addition of

Ag+. However the addition of Ag+ had no effect on the melting

temperature of oligo-2; it remained at 21 �C with Ag+. This

difference was attributed to the different sequence of oligo-1 and

oligo-2. Oligo-1 is a self-complementary oligonucleotide except

that it has five cytosine–cytosine (C–C) mismatch-base pairs,

which can specifically bind with the Ag+ ions to form a C–Ag–C

Fig. 5 Melting curves of oligo-1 and oligo-2 without and with Ag+.

A ¼ [(At�C � A6�C)/(A60�C � A6�C)] at 260 nm,49 a) --- 3.0 mM oligo-1;

b) -C- 3.0 mM oligo-1 in the presence of 9.0 mM Ag+; c) -:- 3.0 mM

oligo-2; d) -+- 3.0 mM oligo-2 in the presence of 9.0 mM Ag+.

2684 | Analyst, 2010, 135, 2682–2687

base pair. So, with the help of Ag+ ions, the oligo-1 can overcome

the negative effect of the C–C mismatched base pairs, and form

a stable duplex structure.48,49

Optimization of the experimental conditions

The effect of pH on the LSPR light-scattering intensity was

studied within the range of pH 4.0–10.0 (Fig. 6). It was demon-

strated that the DI increased when the pH increased from 4.0 to

5.0, this could be attributed to two factors: firstly, the AuNPs

may aggregate in a strong acidic or alkaline medium; secondly,

the double helix structure of DNA is unstable at lower pH

environment. The DI reached a maximum when the solution

pH increased from 6.0 to 8.0. However the DI decreased when the

pH was changed from 8.0 to 10.0, this may be attributed to

the formation of silver hydroxide, which is unfavorable for the

formation of a C–Ag–C base pair, and further blocks the duplex

structure formation. Therefore, a pH 7.0 buffer was employed.

The enhancement of the LSPR light-scattering intensity was

mainly due to the aggregation of AuNPs, thereby the concen-

tration of AuNPs played a crucial role in the research. The effect

of the AuNPs concentration was studied within the range of

0.45–0.95 nM (Fig. 7). DI increased with the increase in the

concentration of AuNPs within the range of 0.45–0.75 nM, and

reached a plateau at a concentration range of 0.75–0.90 nM.

Therefore, a AuNPs concentraton of 0.78 nM was used for the

research.

The effect of oligo-1 concentration on the enhancement of the

LSPR light-scattering intensity was also investigated. In Fig. 8,

the DI increases with the increase of the concentration of oligo-1

within the range of 0.20–0.45 mM, and reaches a maximum and

remains at a plateau when the concentration of the oligo-1 is

higher than 0.45 mM. This suggests that the physisorption of

oligo-1 could protect the AuNPs from aggregation, and reaches

saturation when oligo-1 is higher than 0.45 mM, excessive oligo-1

does not further affect the DI. So 0.5 mM oligo-1 was used

throughout the research.

Naked AuNPs and ssDNA adsorbed AuNPs have different

abilities for NaNO3 resistance;20 whether the AuNPs aggregate

or not is controlled by the concentration of NaNO3. Fig. 9

demonstrates the effect of the concentration of NaNO3. DI

increases dramatically with the increase of the concentration of

Fig. 6 Effect of pH value on the enhancement of the LSPR light-scat-

tering intensity. AuNPs: 0.78 nM; oligo-1: 0.5 mM; Ag+: 0.5 mM; NaNO3:

0.02 M; absorbed time: 10.0 min; desorbed time: 5.0 min.

This journal is ª The Royal Society of Chemistry 2010

Fig. 7 Effect of the concentration of AuNPs on the enhancement of the

LSPR light-scattering intensity. Oligo-1: 0.5 mM; Ag+: 0.5 mM; NaNO3:

0.02 M; pH: 7.0; adsorption time: 10.0 min; desorption time: 5.0 min.

Fig. 8 Effect of the concentration of oligo-1 on the enhancement of the

LSPR light-scattering intensity. AuNPs: 0.78 nM; Ag+: 0.5 mM; NaNO3:

0.02 M; pH: 7.0; adsorption time: 10.0 min; desorption time: 5.0 min.

Fig. 9 Effect of the concentration of NaNO3 on the enhancement of the

LSPR light-scattering intensity. AuNPs: 0.78 nM; oligo-1: 0.5 mM; Ag+:

0.5 mM; pH: 7.0; adsorption time: 10 min; desorption time: 5 min.

Fig. 10 Effect of the adsorption and desorption time on the enhance-

ment of the LSPR light-scattering intensity. AuNPs: 0.78 nM; oligo-1:

0.5 mM; Ag+: 0.5 mM; NaNO3: 0.02 M.

Fig. 11 Selectivity of the assay for the determination of Ag+ ions.

DI ¼ IM�I0. The concentration of all the metal ions was 0.5 mM. AuNPs:

0.78 nM; oligo-1: 0.5 mM; oligo-2: 0.5 mM; NaNO3: 0.02 M; pH: 7.0.

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NaNO3 in the range of 0.005–0.015 M, and it reaches a plateau

when the concentration of NaNO3 varies within the range of

0.015–0.025 M. Excessive NaNO3 concentration induces the

decrease of DI. These results indicate that the AuNPs–oligo-1

could not aggregate completely when the NaNO3 was lower than

the threshold of 0.015 M in the presence of Ag+. But excessive

NaNO3 might lead to the aggregation of AuNPs–oligo-1 in the

absence of Ag+, which results in the decrease of DI at concen-

trations of NaNO3 higher than 0.025 M. Thereby 0.020 M

NaNO3 was applied in the research.

This journal is ª The Royal Society of Chemistry 2010

The LSPR light-scattering enhancement involves the adsorp-

tion of oligo-1 on the surface of AuNPs and the desorption from

it after the addition of Ag+; effects of the adsorption and

desorption time are shown in Fig. 10. The adsorption process

needs 10.0 min so that the enhancement of the LSPR light-

scattering intensity can balance and reach a plateau. After the

addition of Ag+, a duplex structure formed because of the specific

recognition of C–C mismatch between two oligo-1, so that the

oligo-1 desorbed from the surface of AuNPs. This desorption

process needs 5.0 min. Therefore, 10.0 min was used as the

adsorption time and 5.0 min was used as the desorption time.

Calibration curve and accuracy

Using the standard procedure, the relationship between enhanced

LSPR light-scattering intensity and the concentration of Ag+ ions

was investigated. The enhanced intensity was linear with the

concentration of Ag+ within the range of 0.13–1.12 mM, the

regression equation was DI¼ 344.32C + 8.64 (C: mmol L�1), with

a correlation coefficient of 0.9978. The limit of detection was

62.0 nM calculated with the formula CDL¼ 3d/S (where CDL is the

limit of detection, d represents the standard deviation, and S is the

slope of the calibration plot). The relative standard deviation was

1.46% for 1.0 mM Ag+. The linear equation for the colori-

metric method was A ¼ 0.142C + 0.026 (C: mmol L�1,

A¼A700/A520) over the concentration range of 1.29–8.0 mM, with

a correlation coefficient of 0.9912 and a detection limit of 0.87 mM.

Analyst, 2010, 135, 2682–2687 | 2685

Table 1 Results for the determination of of Ag+ in samples

Sample

Found by LSPRlight-scattering(n ¼ 6, mM)

RSD(%, n ¼ 6)

Added/mM

Recovery(%, n ¼ 6)

Found byICP-AES(n ¼ 4, mM)

t-test(t0.95, 5 ¼ 2.57)

Water sample 1 106.8 1.01 0.2 100.2 104.3 1.40Water sample 2 101.0 2.24 0.2 107.7 103.8 0.72Water sample 3 106.7 2.06 0.2 99.4 108.5 0.49Solder sample 1 11.4 5.17 0.2 100.4 11.3 0.01Solder sample 2 10.4 4.51 0.2 98.4 11.2 0.11

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Selectivity of the assay

In addition to sensitivity, we investigated the selectivity of the

assay by comparing the DI intensity in the presence of Ag+ and

other metal ions at a concentration of 5.0 mM. 12 different metal

ions (Cu2+, Co2+, Al3+, Ba2+, Co2+, K+, Zn2+, Ca2+, Cd2+, Fe2+,

Mg2+ and Ni2+) were chosen to study the ion selectivity. Under

the same conditions, only Ag+ showed significant LSPR light-

scattering enhancement (Fig. 11), which proved the high selec-

tivity of the specific binding of Ag+ with the C–C mismatched

base pair between two oligo-1 sequences.18

Determination of Ag+ in samples

Finally, we explored the possibility of using the assay for

detecting Ag+ in synthetic water samples and solder samples

(Table 1). The recovery of this method was 98.4–107.7%, the

determination results of five samples were in accordance with

that of the inductively coupled plasma-atomic emission spec-

troscopy (ICP-AES) method, the t-test showed no salience

difference between these two methods.

Conclusion

A novel localized surface plasmon resonance light-scattering

method for determining Ag+ with unmodified Au nanoparticles

was established. Single strand oligo-1 adsorbs on the surface of

AuNPs and protects them from aggregation with the addition of

NaNO3. Due to the specific binding property of Ag+ with C–C

mismatched base pairs, hybridization occurred with oligo-1, and

it formed a duplex structure with the addition of Ag+, and then

was desorbed from the surface of AuNPs as a result. This process

was accompanied by an enhancement of localized surface plas-

mon resonance light-scattering, a decrease of surface plasmon

absorption band at 520 nm and a color change change of the

solution from red to purple. Under the optimum conditions, the

enhanced intensity of localized surface plasmon resonance light-

scattering was proportional to the concentration of Ag+ ions in

the range of 0.13–1.12 mM, with a limit of detection of 62.0 nM.

Satisfactory results were obtained when the assay was applied to

the determination of Ag+ in the water samples and industrial

products.

Acknowledgements

This work was supported by the Natural Science Foundation of

China (No: 20975116) and SRF for ROCS, SEM.

2686 | Analyst, 2010, 135, 2682–2687

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