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A simple, reliable and sensitive colorimetric visualization of melaminein milk by unmodified gold nanoparticles

Hong Chi,ab Bianhua Liu,a Guijian Guan,a Zhongping Zhang*ab and Ming-Yong Han*c

Received 6th January 2010, Accepted 2nd February 2010

First published as an Advance Article on the web 12th February 2010

DOI: 10.1039/c000285b

In this paper, we report a simple, reliable and sensitive colourimetric visualization of melamine in milk

products using citrate-stabilized gold nanoparticles (Au NPs). Upon exposure to ppb-level melamine,

gold nanoparticle solution exhibits a highly sensitive colour change from red to blue and rapid

aggregation kinetics within the initial 5 min, which can directly be seen with the naked eye and monitored

by UV-vis absorbance spectra. As confirmed by the comparison with six other typical amino compounds,

the melamine molecule itself contains multiple strong-binding sites to the surface of Au NPs and thus

plays a role of molecular linker to efficiently crosslink Au NPs. Further evidence is that the sensitivity is

significantly improved when NaHSO4 is added to promote the ligand exchange between citrate and

melamine at the surface of Au NPs. The NaHSO4-optimized Au NPs system provides a rapid

colourimetric assay for the rapid detection of melamine down to �25 ppb in real milk products.

1. Introduction

Because of the high nitrogen content (66% by mass), the illegal

addition of melamine (1,3,5-triazine-2,4,6-triamine) into milk

products by unethical producers as an unacceptable non-protein

N source, can give an incorrect high readout of apparent protein

content when conventional standard Kjeldahl or Dumas tests are

performed to estimate protein levels. Recently, high levels of

melamine were reported in milk and various dairy products,

which have caused the renal failure and even death of infants in

China in 20081 and killed thousands of cats and dogs in the U.S.

in 2007.2 For the accurate detection of melamine in food, the

currently used methods are usually time-consuming and

cumbersome with the employment of expensive enzyme-linked

immunoassays3 or complicated instruments4–9 such as liquid

chromatography,4 gas chromatography,5,6 low-temperature

plasma probes coupled with tandem mass spectrometry,7

surface-desorption ionization mass spectrometry8 and surface-

enhanced Raman spectrometry.9 Until now, it still remains

a great challenge to develop a simple but reliable detection

method for a rapid and sensitive melamine colourimetric visu-

alization assay without the use of costly instrumentation or the

need for tedious training.

As a result of surface plasma resonance, gold nanoparticles

(Au NPs) exhibit size-tunable optical properties.10 Colloidal Au

NPs are wine red, whereas their aggregates appear purple or blue

coloured. The controlled colour change induced by aggregation,

the basis of colourimetric assay, has been widely used in chemo/

biosensors for the detection of various analytes such as protein,11

DNA,12 metal ions13 and small molecules.13,14 Usually, colloidal

aInstitute of Intelligent Machines, Chinese Academy of Sciences, Hefei,Anhui, 230031, China. E-mail: zpzhang@iim.ac.cnbDepartment of Chemistry, University of Science and Technology of China,Hefei, Anhui, 230026, ChinacInstitute of Materials Research and Engineering, A-STAR, 3 ResearchLink, Singapore 117602. E-mail: my-han@imre.a-star.edu.sg

1070 | Analyst, 2010, 135, 1070–1075

Au NPs were chemically modified with antibodies, oligonucleo-

tides and other ligands for specific recognition to target analytes.

In such a way, the surface-functionalized Au NPs can be bound

together by analytes to form aggregated Au NPs accompanying

clear colour changes. In contrast, we found that the melamine

molecule itself contains multiple amino ligands with a very

strong binding ability to the surface of Au NPs. Au NPs can thus

be crosslinked directly in the presence of certain amounts of

melamine without any extra aid such as specific receptors and the

in-advance addition of analyte. The resulting aggregation-based

change in colour can be developed into a simple and reliable

colourimetric detection of ppb-level melamine in milk as

measured with the naked eye or UV-vis spectroscopy.

2. Experimental

2.1 Materials

Melamine (purity >99%, Alfa-Aesar), 6-methyl-1,3,5-triazine-

2,4-diamine, 2-amino-4,6-dimethypyrimidine and 1,2-cyclo-

hexanediamine (Sigma-Aldrich) were used as received.

Chloroauric acid tetrahydate (HAuCl4$4H2O), trisodium citrate

dehydrate, n-butylamine, cyclohexylamine and trichloro acetic

acid were purchased from Sinopharm Chemical Reagent Co.,

Ltd. Solid phase extraction cartridge PCX-SPE (60 mg, 3 mL)

was supplied by Agela Cleanert.

2.2 Synthesis of Au NPs

Au NPs were prepared by the reduction of HAuCl4 with trisodium

citrate.15 Typically, 25 mL of trisodium citrate (38.8 mM) was

rapidly injected into a boiling solution of HAuCl4 (250 mL, 1 mM),

and the mixed solution was further refluxed for another 15 min

into a wine-red suspension. The suspension was gradually cooled

to room temperature under stirring, and then filtered through

a 0.2 mm Millipore membrane. The filtrate was stored in a refrig-

erator at 4 �C for further use. The size of Au NPs is �13 nm as

This journal is ª The Royal Society of Chemistry 2010

Fig. 1 The interaction of melamine with Au NPs and the aggregation of

citrate-stabilized Au NPs through direct crosslinkage with melamine.

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examined by transmission electron microscope.

The concentration of the Au NPs as determined by UV-vis

spectrometry was 10 nM.

2.3 Detection of melamine

Typically, 10 mL of Au NPs suspension was diluted with 50 mL

of deionised water to give a total volume of 60 mL as a stock

liquid for the detection of melamine. Different amount of analyte

was added into 2 mL of the above Au NPs suspension. The

colour change and absorbance spectra were observed with the

naked eye and recorded with UV-vis spectrometer, respectively.

The aggregating kinetics of Au NPs at different concentrations of

melamine analyte was obtained by the measurements of absor-

bance spectra at the interval of 1.5 min. On the other hand, the

sensitivity of Au NPs suspension to melamine was further opti-

mized with NaHSO4. Typically, 10 mL of original Au NPs

suspension was diluted with 49.52 mL of deionised water and

0.48 mL of 0.1 M NaHSO4 to give the mixture with a total

volume of 60 mL. The Au NPs suspension containing 0.8 mM

NaHSO4 was used as a stock liquid for the detection of melamine

in real milk sample.

2.4 Extract of real sample

The infant milk powder without melamine was bought from

a local supermarket and was directly used. 1.0 wt & melamine was

doped into the infant milk products for validating the effective-

ness of the colorimetric detection of melamine in real sample. The

extraction of milk powder for the determination of melamine was

carried out according to the method stipulated by the National

Standard of China (GB/T22388-2008). Typically, 2 g milk powder

and 2 mg of melamine were mixed into 15 mL of 1.0 wt % trichloro

acetic acid. Then, 5 mL of acetonitrile was added into the above

mixture. After ultrasonication for 20 min, the mixture was

centrifuged at 10 000 rpm for 10 min. The supernatant was filtered

through the filter paper wetted with 1.0% trichloro acetic acid in

advance, and the filtrate was diluted by 1.0% trichloro acetic acid

to give a total volume of 25 mL. Further purification by solid

phase extraction cartridge (Agela Cleanert PCX-SPE, 60 mg,

3 mL) was carried out as follows: The cartridge was first condi-

tioned with 3 mL of methanol and then 5 mL of deionised water.

5 mL of the sample solution was first diluted with 5 mL of water

and was passed through the cartridge. After successive washing

with 3 mL of water and 3 mL of methanol, the cartridge was dried

under negative pressure for 3 min, and finally eluted with 6 mL of

5% ammonium hydroxide in methanol with a flow rate slower

than 1 mL min�1. The eluent was collected and dried under

nitrogen at 50 �C. The resultant residue was redissolved in 4 mL of

deionised water. The solution of residue was filtered through

a 0.2 mm PVDF filter membrane to obtain the final sample solu-

tion. The extract contained 80 ppm melamine by the measurement

of high-performance liquid chromatography, and was diluted to

5 ppm for the colorimetric assay.

2.5 Characterizations

UV-vis spectra were recorded with a Shimadzu UV-2550 spec-

trophotometer using 1-cm path length quartz cuvettes. The Au

NPs were observed by JEOL 2010 transmission electron

This journal is ª The Royal Society of Chemistry 2010

microscope. The optical photographs were taken with Sony

digital camera. The concentration of melamine in the sample

solution was measured by Waters Module-600 high-performance

liquid chromatography with UV 996 detector.

3. Results and discussion

3.1 Molecular linker-based aggregation mechanism

The surface adsorption of electron-rich nitrogen-containing

ligands on Au NPs has been well documented in the literature16–20

including our previous work.16 Primary amines with electron-rich

nitrogen atoms are more likely to be bound onto the surface of

metal nanoparticles through the coordinating interactions with

the electron-deficient surface of metal nanoparticles.17,18 In

particular, the ring nitrogen of hybrid aromatics exhibits much

stronger binding ability/affinity to Au NPs and therefore the

pyridine-like compounds are often used as transfer agents of Au

NPs from one phase to another,20 or as a mediator of aggregating

state of metal nanoparticles in surface-enhanced Raman spec-

troscopic assay.17,18 Accordingly, melamine with multiple

binding sites including three exocyclic amino groups and a three-

nitrogen hybrid ring may strongly coordinate to Au NPs by the

ligand exchange with weakly surface-bound citrate ions, and

finally crosslink Au NPs. The colloidal stability is drastically

reduced to result in the fast/prompt occurrence of particle

aggregation, as revealed in Fig. 1. The molecular linker-based

aggregation offers a possible approach to a simple and rapid

colourimetric assay for the detection of melamine in milk prod-

ucts, which does not require any extra aid such as specific

acceptors.

Recently, Lu et al.21 reported the colourimetric detection of

melamine in milk by using triazinane-modified Au NPs through

the triple hydrogen-bond recognition. In order to show the high

sensitivity of colourimetric assay, they intentionally added 1 mM

melamine in advance in the modified Au NPs suspension.

However, using the solution stabilized with 1 mM melamine to

assay the melamine with the concentration in the several ppb

(nano M) range is similar to the use of buffer solution to measure

pH data in solutions.22 Therefore, the colourimetric assay

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seriously lacks reliability, repetition and accuracy in principle,

and just conceals the fact that the modified Au NPs themselves

are not sensitive to melamine as low as 1 mM (126 ppb). On the

other hand, in order to demonstrate the effectiveness of the

colourimetric assay, they doped milk samples with melamine in

a 1 : 1 ratio (wt/wt), which would dilute the concentrations of

residual amino acids and other impurities in samples. This is

impracticable because so high a melamine content never happens

in any real milk products.

Fig. 3 The corresponding plot of A640/A520 versus melamine concen-

tration from 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4

to 1.5 mM. All data were collected after 5 min. Arrow indicates mutation

point of colour.

3.2 Colourimetric sensitivity of Au NPs suspension to trace

melamine

In this study, 13-nm Au NPs were first synthesized by the

reduction of HAuCl4 with sodium citrate in aqueous solution.15

The resulting citrate-stabilized Au NPs were wine red, because of

their strong surface plasma resonance at 520 nm. Upon the direct

exposure of citrate-stabilized Au NPs to melamine, the colour of

Au NPs changed from wine red, purple to blue progressively

while melamine concentrations increased from 0 to 1.5 mM

(Fig. 2A). Clear colour changes were observed from wine red to

deep red at a melamine concentration as low as 0.6 mM

(i.e., 75 ppb). Meanwhile, the melamine-induced aggregation of

Au NPs was also monitored by UV-vis spectroscopy (Fig. 2B).

With the addition of melamine from 0 to 1.5 mM, the original

absorbance of Au NPs at 520 nm decreased gradually while

a new absorbance (centered at �640 nm) from their resulting

aggregates increased obviously. This indicates that more and

more Au NPs were consumed to form more and more aggregates,

which was further confirmed by the TEM observations: the

monodisperse nanoparticles in the absence of melamine (Fig. 2C)

and the significant aggregation of the nanoparticles in the pres-

ence of 1.1 and 1.5 mM melamine (Fig. 2D and 2E, respectively).

These above observations clearly indicate that trace melamine

can directly induce the aggregation of unmodified Au NPs, and

the complex triazinane modification at the surface of Au NPs, as

Fig. 2 (A) Visual colour change of Au NPs with the indicated concen-

trations of melamine. (B) The evolution of UV-vis absorbance spectra of

Au NPs suspension with the melamine concentration from 0, 0.1, 0.2, 0.3,

0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4 to 1.5 mM. TEM images of

Au NPs with addition of (C) 0, (D) 1.1, and (E) 1.5 mM melamine,

respectively.

1072 | Analyst, 2010, 135, 1070–1075

reported by Lu et al.,21 did not increase their aggregation sensi-

tivity to melamine.

Furthermore, the corresponding colourimetric effect was

evaluated by comparing the A640/A520 values in the presence of

different concentrations of melamine (Fig. 3). The extinct/very

small ratios reveal slight visible colour change from wine to deep

red in the melamine concentration range of 0.6–0.9 mM, and the

large ratios show colour mutation from deep red to blue with the

further increase of melamine. The extinct ratios slightly increased

from 0.07 to 0.13 in the melamine concentration range of

0–0.9 mM. The ratios of A640/A520 for 1.0, 1.2 and 1.3 mM

melamine are 0.18, 0.7 and 0.92, respectively. As indicated by the

arrow in Fig. 3, the mutation point of colour is at �0.9 mM

melamine. The measurements of extinct ratios further confirm

that particle aggregation and corresponding colour change occur

before melamine concentration rises up to 1.0 mM.

3.3 Aggregation kinetics of Au NPs with melamine

We examined the aggregation kinetics of unmodified Au NPs in

the presence of trace melamine by measuring the temporal

evolution of A640/A520 at the interval of 1.5 min. Because the high

melamine concentration (>1.5 mM) leads to the rapid precipita-

tion of Au NPs during the mixing of melamine with Au NPs, the

kinetic aggregation can not be accurately characterized by

UV-vis spectroscopy. Therefore, we chose four typical concen-

trations of melamine (0.6, 1.0, 1.1 and 1.2 mM) for the evaluation

of aggregation kinetics. As shown in Fig. 4, the higher the

concentration of melamine is, the faster the extinct ratio rises in

the initial stage. At the concentration of 1.2 mM melamine, the

extinct ratio exhibits a rapid increase from original 0.07 to 0.67

during the first 4.5 min, followed by a slow increase to

a maximum value of �0.93 after 20 min, revealing that most of

the free monodisperse Au NPs in suspension are promptly

consumed in the initial stage. The inset of Fig. 4 shows that the

wine red of Au NPs promptly changes into light purple after

1 min, purple after 5 min and finally blue after 20 min. The

corresponding extinct ratios are about 0.32, 0.7 and 0.87,

respectively.

This journal is ª The Royal Society of Chemistry 2010

Fig. 4 The plots of A640/A520 versus time at different melamine

concentrations (the data were taken at the interval of 1.5 min). Inset

images show the colour change of Au NPs suspension with time after the

addition of 1.2 mM melamine.

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In the case of 1.0 mM melamine, however, the extinct ratio

slightly increases from �0.07 to 0.15 in the first 3 min and then

exhibits a linear enhancement to 0.36 in the next 45 min, sug-

gesting a slow crosslinking aggregation of Au NPs. When the

concentration of melamine was further reduced to 0.6 mM, the

extinct ratio will very slightly increase in the first 1.5 min and

then almost keeps at a small constant value, in which the free

melamine molecules were almost exhausted in a shorter time but

is not enough to cause the significant aggregation of Au NPs. In

general, the aggregation of Au NPs starts from the exchange

between melamine and citrate ligands at the surface of Au NPs,

followed by the further crosslinkage among Au NPs through the

multiple binding sites of melamine molecules. Free melamine

molecules are promptly exhausted at the concentrations lower

than 1.0 mM due to the strong adsorption at the surface of Au

NPs. In contrast, free Au NPs in suspension were promptly

consumed when the concentration of melamine is higher than

1.1 mM. These detailed observations further suggest that mela-

mine has a strong binding and crosslinking ability to Au NPs

without the aid of recognition acceptor.

Fig. 5 (A) The colours of unmodified Au NPs suspension (in Microlon ELIS

concentrations. (B) The corresponding A640/A520 ratios of Au NPs suspensio

This journal is ª The Royal Society of Chemistry 2010

3.4 Molecular selectivity of Au NPs to seven amino compounds

To better understand the melamine-induced aggregation mech-

anism, we compared the colorimetric selectivity of citrate-stabi-

lized Au NPs to seven amino compounds in Microlon ELISA

Plates 96 (Fig. 5A). For the three compounds with one primary

amino group, no visible colour change of Au NPs was observed

with the addition of 10 mM ammonium hydroxide (a), and light

purple was formed with the addition of 10 mM n-butylamine

(b) and cyclohexylamine (c), respectively. In contrast, the addi-

tion of 0.1 mM 1,2-cyclohexanediamine (d) with two primary

amino groups can result in a purple colour. Furthermore, the

amino compounds with pyrimidine (e) and triazine (f) rings at the

concentrations of 0.1 mM and 10 mM can transform the wine red

of Au NPs into blue and deep purple, respectively. Meanwhile,

only 1.0 mM of melamine (g) is sufficient to transform the wine

red of Au NPs into purple promptly. Therefore, the sensitivity of

Au NPs to the above compounds is in the order of g > f > e >

d [ c z b > a. It is clear that the high sensitivity of Au NPs to

melamine is attributed here to the multiple coordinating inter-

actions to crosslink Au NPs strongly via three exocyclic amino

groups and three ring nitrogen atoms with Au NPs, as illustrated

in Fig. 1.

The molecular selectivity was further evaluated by testing the

UV-vis spectroscopic response of Au NPs to the above seven

compounds of 1.5 mM in solution (Fig. 5B). The extinct/very

small A640/A520 ratios for a, b, c, d and e are almost identical to

that of blank Au NPs (< 0.1), suggesting no obvious aggregation

of Au NPs. Melamine exhibits the highest A640/A520 value (1.29),

which is 2.5 fold more when compared with its most similar

compound f (0.48). These quantitative measurements further

show that both the exocyclic amino groups and the ring nitrogen

atoms in melamine induce the aggregation of Au NPs coopera-

tively, as drawn in Fig. 1.

3.5 Optimized sensitivity with sodium bisulfate through surface

ligand exchange

In order to further confirm the molecular-linker-based aggrega-

tion mechanism and to improve the colorimetric sensitivity, we

tested the effect of NaHSO4 as a promoter of ligand exchange

A Plates 96) after the addition of seven amino compounds with different

n with the addition of 1.5 mM analytes.

Analyst, 2010, 135, 1070–1075 | 1073

Fig. 7 UV-vis spectra of the NaHSO4-optimized Au NPs sensor (2 mL)

after the addition of 0, 5, 10, 20, 30, 40, 50, 100 mL of the extracts from

(A) blank raw milk and (B) melamine-containing milk (the extract

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between citrate and melamine at the surface of Au NPs.

Although the citrate ligands introduced in the synthesis can

weakly bind onto the surface of Au NPs as a stabilizer,16 its

relatively-large molecular size may hinder the adsorption of

melamine to Au NPs and the further crosslinkage to Au NPs. It

has well been known that SO42� anions at a concentration higher

than 0.1 M can destroy the stability of citrate-capping metal NPs

colloids and result in the aggregation of metal NPs,23 which may

be achieved by occupying the surface of metal NPs through the

exchange with the weakly bound citrate ligands.24 The acidity of

HSO4� salt further promotes the exchange between SO4

2� anions

and citrate ligands due to the weak basicity of citrate. However,

the SO42� anions at the concentration as low as 1.0 mM does not

result in the aggregation of Au NPs at all, but may promote the

ligand exchange between other strongly-bound ligands and

citrate ligands at the surface of metal NPs. It is thus expected that

the sensitivity of Au NPs to melamine will be improved by the

addition of suitable amount of NaHSO4 in Au NPs suspension.

Fig. 6A shows the absorbance spectra of Au NPs suspension

before and after the addition of 0.8 mM NaHSO4. The two

absorbance spectra are completely identical, suggesting no

aggregation of Au NPs. Moreover, the ratio A640/A520 for the Au

NPs suspension with 0.8 mM NaHSO4 keeps at a constant value

with time (inset of Fig. 6A). These clearly indicate that Au NPs

suspension is still highly stable after the addition of 0.8 mM

NaHSO4. We further compared the sensitivities of Au NPs to

melamine before and after the addition of NaHSO4 by measuring

the A640/A520 at different analyte concentrations. As shown in

Fig. 6B, the A640/A520 ratios for NaHSO4-optimized Au NPs in

the presence of 0.3–1.1 mM melamine are obviously larger than

the corresponding ratio for original Au NPs. Moreover, the

difference becomes larger with the increase of melamine

concentration in this range as indicated with upright lines in

Fig. 6B. For example, while the A640/A520 ratios at 0.5, 0.7 and

0.9 mM melamine for original Au NPs are only 0.09, 0.11 and

0.13, respectively, the corresponding ratios for NaHSO4-opti-

mized Au NPs are 0.16, 0.29 and 0.50, respectively. That is to say,

the extinct ratio was enhanced �2–4 fold at the low concentra-

tion range of analyte by the addition of NaHSO4. Therefore, the

addition of NaHSO4 significantly improves the aggregating

sensitivity of Au NPs to trace melamine.

Fig. 6 Optimized sensitivity of colloidal Au NPs with NaHSO4.

(A) Absorbance spectra of colloidal Au NPs before (solid line) and after

(dashed line) the addition of 0.8 mM NaHSO4 (inset is the temporal

evolution of A640/A520 values after the addition of 0.8 mM NaHSO4).

(B) The evolutions of A640/A520 values of colloidal Au NPs with different

melamine concentrations before (a) and after (b) the addition of 0.8 mM

NaHSO4.

1074 | Analyst, 2010, 135, 1070–1075

3.6 Detection of melamine in real samples

To demonstrate whether the citrate-stabilized Au NPs can be

used for the direct detection of melamine in milk powder, we

doped the infant milk with 1.0 wt & melamine, and the milk

powder was extracted according to the method stipulated by the

National Standard of China (GB/T22388-2008). The extract

contains 85 ppm melamine with a recovery ratio of 85% from the

as-added melamine, and was diluted to 5 ppm for the colori-

metric assay. Meanwhile, the blank extract was also obtained by

the identical procedure for evaluating the reliability of the

colorimetric assay. UV-vis spectra confirm that the NaHSO4-

optimized Au NPs are highly sensitive and selective to the

extracts of milk powder. With the addition of blank extract in the

Au NPs suspension, the absorbance at 520 nm is lightly reduced,

but the absorbance at 640 nm showed no increase (Fig. 7A),

indicating that the aggregation of Au NPs did not significantly

occur and the decrease of absorbance at 520 nm mainly resulted

from the dilution of Au NPs after the addition of the blank

extract. With the addition of the extract from melamine-con-

taining milk, however, the absorbance at 520 nm obviously

reduced and the new absorbance at 640 nm violently increased

with the amount of extract (Fig. 7B). When the final concen-

tration of melamine reached 240 ppb, the two absorbances at 520

and 640 nm were almost equal. As shown in Fig. 8, while the

corresponding colour of Au NPs keeps constant with the addi-

tion of blank extract (upper image), we can see with the naked

eye that the colour of Au NPs with the addition of real sample

contains 5 ppm melamine; final concentrations are 0, 12.5, 25, 50, 75, 100,

120, 240 ppb, respectively). The data were collected after 5 min.

Fig. 8 Visual colour changes of the NaHSO4-optimized Au NPs sensor

after the addition of the extracts from (upper) blank raw milk and

(bottom) melamine-containing milk (the indicated amounts of the extract

correspond to the final melamine concentrations: 0, 12.5, 25, 50, 75, 100,

120, 240 ppb, respectively).

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changes from original wine red to blue after 5 min (bottom

image, Fig. 8). The colorimetric assay allows a detection

concentration as low as 25 ppb (i.e., 0.2 mM) for a rapid and

reliable visualization of melamine by the comparison with blank

sample.

4. Conclusions

In summary, a simple and reliable colorimetric detection of ppb-

level melamine in milk products has been demonstrated by the

direct use of as-prepared or optimized citrate-stabilized gold

nanoparticles. Due to the multiple strong binding sites to the

surface of Au NPs melamine, can serve as a molecular linker to

directly induce the aggregation of the unmodified Au NPs

without the aid of specific acceptor. The resultant colour change

from red to blue forms the basis of a simple-but-sensitive

colorimetric assay for the visualization of melamine in milk

products. The NaHSO4-optimized Au NPs sensor allows the

rapid detection of melamine down to �25 ppb level within 5 min

and can tolerate the interference of other impurities such as

residual nitrogen-containing compounds. The technique repor-

ted here is also suitable when starting with various formulas of

milk products such as liquid milk.

Acknowledgements

This work was supported by Natural Science Foundation of

China (No. 20925518, 20875090, 20807042, 30901008) and

China-Singapore Joint Project (2009DFA51810) and 863 project

of China (2007AA10Z434) and Innovation Project of Chinese

Academy of Sciences (KSCX2-YW-G-058).

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