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Spectrochimica Acta Part A 88 (2012) 97– 101

Contents lists available at SciVerse ScienceDirect

Spectrochimica Acta Part A: Molecular andBiomolecular Spectroscopy

j ourna l ho me page: www.elsev ier .com/ locate /saa

tudy of the surface-enhanced Raman spectroscopy of residual impuritiesn hydroxylamine-reduced silver colloid and the effects of anions on theolloid activity

iao Dong, Huaimin Gu ∗, Fangfang LiuOE Key Laboratory of Laser Life Science & Institute of Laser Life Science, College of Biophotonics, South China Normal University, 510631 Guangzhou, China

r t i c l e i n f o

rticle history:eceived 18 July 2011eceived in revised form0 November 2011ccepted 4 December 2011

eywords:g nanoparticlesaman spectroscopy

a b s t r a c t

The paper investigated the residual ions in hydroxylamine-reduced silver colloid (HRSC) and the rela-tionship between the condition of HRSC and the enhanced mechanisms of this colloid. We also detectedthe SERS of MB and studied the effects of anions on the Raman signal. In the case of HRSC, the bands ofresidual ions diminish while the bands of Ag-anions increase gradually with increasing the concentra-tions of Cl− and NO3

−. It means the affinity of residual ions on the silver surface is weaker than that ofCl− and NO3

− and the residual ions are replaced gradually by the added Cl− or NO3−. The Raman signal of

residual ions can be detected by treatment with anions that do not bind strongly to the silver surface, suchas SO4

2−. The most intense band of Ag-anions bonds can be also observed when adding weakly binding−

ydroxylamine-reducedggregating agentsffinityesidual ions

anions to the colloid. However, the anions which make up the Ag-anions bonds are residual Cl and theeffect of weakly binding anions is only to aggregate the silver particles. Residual Cl− can be replaced byI− which has the highest affinity. From the detection of methylene blue (MB), the effects of anions onthe enhancement of Raman signal are discussed in detail, and these findings could make the conditionssuitable for detecting analytes in high efficiency. This study will have a profound implication to SERSusers about their interpretation of SERS spectra when obtaining these anomalous bands.

. Introduction

Since the surface-enhanced Raman spectroscopy (SERS) tech-ology was discovered thirty years ago, it has gained widespreadttention from the research community [1–10]. The enhancementf Raman signal in SERS is closely related to the SERS-active sub-trates which are various sorts of metallic structures in nanoscale11–14]. The metals that were used as SERS-active substratencludes Ag, Au, Cu, Na, Li, and other transition metals [15]. Amongarious sorts of SERS-active substrate, metal colloids have becomehe most commonly used Raman active substrate for their sim-le preparation and notable effect [16]. Although there are a largemount of literatures in the supplement of enhancement mech-nisms of silver colloids and the optimization of experimentalondition, the properties and the enhancement mechanisms areot completely clear yet.

It is well known that silver colloid has the largest enhancement

ffect among metal colloids, so in this paper, hydroxylamine-educed silver colloid (HRSC) was studied for its fast and simplerocedure at room temperature and stable, highly SERS-active

∗ Corresponding author. Tel.: +86 20 85216972; fax: +86 20 85216052.E-mail address: guhm@scnu.edu.cn (H. Gu).

386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.saa.2011.12.007

© 2011 Elsevier B.V. All rights reserved.

features [13]. Hydroxylamine hydrochloride, sodium chloride,sodium sulfate, sodium nitrate, and sodium iodide were addedrespectively, to investigate the change in signal from Ag colloid,and make further investigation on the aggregation of the colloid andthe modification of the surface chemistry on anions addition. Fromprevious studies [17,18], there are numerous investigations on thechange in signal from Ag colloids when adding different anionsas aggregating agents. Recently, Bell and Sirimuthu studied therole of anions to citrate-reduced silver colloid [15]. Alvarez-Pueblaand his coworkers studied the role of silver nanoparticle surfacecharge in SERS [19]. Culha and his coworkers studied the relation-ship between the size of silver nanoparticles and the SERS activity[20]. To our knowledge there are no published studies on the SERSspectra of HRSC when adding a great range of salts as aggregatingagents, and there are few reports on the relationship between thecondition of HRSC and the enhanced mechanisms. There were somenew phenomena in this investigation and they were given reason-able explanation. The results show that the residual ions could notbe replaced by sulfate anions and it can be detected easily. Whenadding chloride or nitrate to the colloid, the bands of impurities

are not distinct and are replaced by the anomalously intense band,which is assigned to the Ag-anions bonds. In this paper, the spectraof HRSC were also detected at different time periods to investi-gate the changing rule of spectra of residual ions with time. The

9 ica Acta Part A 88 (2012) 97– 101

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Fig. 1. UV-vis absorption spectrum of the hydroxylamine-reduced silver colloid

8 X. Dong et al. / Spectrochim

nalyte considered in this paper is methylene blue (MB), whichields a tiny fluorescence and displays very strong SERS bands [21].t these experimental conditions, MB was chosen as the targetolecule to find the suitable condition for SERS enhancement and

nvestigate the rational principles of the phenomenon. The pur-ose of this study is to further investigate the effect of anions onilver colloid and to make the experimental conditions suitable foretecting some analytes in high efficiency on rational principles. Inddition, this study will have a profound implication to SERS usersbout their interpretation of SERS spectra when obtaining thesenomalous bands.

. Materials and methods

.1. Instrumentation

A 514.5 nm Argon ion laser was used as excitation source. Theaser spectral line width was further spectrally purified with annterference type laser line bandpass filter and a notch filter (HSPF-14.5-1.0, Kaiser Optical Syetemes., Inc., USA) was used to rejecthe excitation light while allowing the Raman signal to enter thepectrometer system.

The Raman system was equipped with a BX-41 Olympus micro-cope and 45× objective lens. The spectrometer system (Actonpectro@2300i, Princeton Acton, USA) was used with a 1200 gr/mmolographic grating. An air-cooled CCD detector (Pixis 256, Prince-on Acton, USA) was used for measuring the Raman signal by anntegration method. The Raman spectrum and absorption spectrum

ere obtained at room temperature. The absorption spectra wereetected by spectrophotometer (Lamda 35, PE company, USA). Theata processing was operated by using Origin 7.5 software.

.2. Materials

Hydroxylamine hydrochloride (HONH3Cl), silver nitrateAgNO3), and MB were purchased from Sigma. Sodium chlorideNaCl), sodium nitrate (NaNO3), sodium sulfate (Na2SO4), sodiumodide (NaI), and sodium hydroxide (NaOH) were purchasedrom Sangon. The tri-distilled water was used for all solutionreparation.

.3. Silver colloid and sample preparation

Silver nitrate (AgNO3) was reduced by hydroxylamineydrochloride (HONH3Cl) (Leopold and Lendl method [13]). 10 mLf a more-concentrated hydroxylamine hydrochloride/sodiumydroxide solution (1.5 × 10−2 M/3 × 10−2 M, respectively) wasdded to 90 mL of a less-concentrated silver nitrate solution1.11 × 10−3 M) rapidly with vigorous stirring. The concentrationf hydroxylamine hydrochloride/sodium hydroxide and silveritrate was chosen to obtain a concentration of 1.5 × 10−3/3 × 10−3

nd 10−3 M in the final reaction mixture. The silver colloid wastored in an amber bottle and was laid in dark place.

The sample that was used for Raman studies was MB. Theoncentration of MB solution was 1.25 × 10−3 mM (to final concen-ration – the concentration of MB after mixed with silver colloid).he solution was stored at ambient temperature for one day to gettable solution.

. Results and discussion

Fig. 1 shows the UV-vis absorption spectra of silver colloid mixedith hydroxylamine hydrochloride and anions. The characteriza-

ion of colloid can be described by its UV-vis absorption spectrumhich is a function of size and shape of particles. The position of

and the spectra when adding uniform concentration of NaCl, NaNO3, Na2SO4, andHONH3Cl respectivelty.

the maximum absorption band and the intensity of the absorp-tion band are highly sensitive to the dimension, distribution andshape of the particles [22]. Fig. 1 shows that the absorption max-imum of HRSC is at 418 nm and the full width at half maximum(FWHM) is about 100 nm. This result indicates a monodispersedsize of the silver particles in this kind of silver colloid. From theliterature [13], a mean diameter of approximately 34 nm was esti-mated. After adding NaCl, NaNO3, Na2SO4, and HONH3Cl to thecolloid several minutes, respectively, the intensity of the absorp-tion band decreases, and its maximum band at 418 nm shift to406 nm, 402 nm, 407 nm, 400 nm, respectively. A new absorptionpeak appears at long wavelength (at about 700–800 nm) whenadding NaNO3, Na2SO4, and HONH3Cl to the colloid. From the pre-vious research, the evolution of the absorption spectrum of silvercolloid can be attributed to the aggregation of the colloidal parti-cles [23,24]. When anions are added to colloid, the energy stateson particle surface are easily influenced by chemical contributionsof the anions and the inter-band transitions between particles viaanions [23]. The shift of particles’ surface energy states inducedby additional anions is considered as energy hybridization effecton particle surface. For this reason the absorption spectra changegreatly in the presence of anions compared with the absence ofanions [23].

Fig. 2 shows the Raman spectra of Ag colloid at different timesafter it is mixed with hydroxylamine hydrochloride. The bands areobserved at 239, 612, 772, 1182, 1362, 1508, 1570, 1648 cm−1 inthe beginning. The specific origins of all of these bands are notyet clear, and they may be ascribed to one or more combinationof anions such as NO3

−, Cl−, and hydroxylamine ions [18]. Theband at 239 cm−1 drops sharply while the other bands increasegreatly in the course of time. In other words, the relative intensitieschange over time. This result is reflected in the 239:1648 cm−1 ratio,which is 1.85, 1.58, 0.61, 0.62, and 0 corresponding to 1 min, 5 min,15 min, 30 min and 1 h after mixing Ag colloid with hydroxylaminehydrochloride. From the previous study, the strong band at about239 cm−1 also appeared for citrate-reduced silver colloid and it wasassigned to Ag-anion bonds [15]. From Fig. 3, the intensity of theAg-anions band increases with the increase of sodium nitrate. How-ever, the bands at 1178, 1362, 1508, 1581, 1648 cm−1 which may

be assigned to some impurities decrease. It implies that in the caseof HRSC, the affinity of the impurities are weaker than that of NO3

−

and the impurities can be removed gradually as the amount of NO3−

increases. The band at 239 cm−1 in Figs. 2 and 3 may be assigned to

X. Dong et al. / Spectrochimica Acta Part A 88 (2012) 97– 101 99

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ig. 2. SERS spectra of hydroxylamine-reduced silver colloid when adding HONH3Clt different times.

he bonds between Ag and residual chloride ions already existingn the hydroxylamine colloid. The adding of hydroxylamine leadso the aggregation of silver particles and yields “hot spots” at andear the touching points of the particles, which can greatly enhancehe signal of impurities and Ag Cl bonds. The impurity bands thatttain maximum intensity are clearly surface-enhanced after mix-ng hydroxylamine hydrochloride with silver colloid 15 min, andhe bands are observed clearly at 239, 313, 352, 399, 520, 569, 612,72, 929, 1128, 1182, 1308, 1362, 1435, 1508, 1570 and 1648 cm−1.owever, these bands nearly disappear 1 h later and this can bettributed to the over-aggregated of silver colloid after long timeixing.Fig. 4 shows a series of Raman spectra of HRSC to which

ncreasing amounts of NaCl was added. It can be seen clearlyhat as the concentration of sodium chloride increases, the inten-ity of the band at 239 cm−1 which is assigned to the Ag-anionsonds increases distinctively, while the bands at 1362, 1508, 1570,648 cm−1 which are assigned to the bands of impurities adsorbingn the silver surface decrease and disappear at the highest concen-ration of sodium chloride (4 × 10−2 mol dm−3). It shows that in thease of HRSC, the affinity of the impurities is wake and they can be

asily replaced by more strongly binding anions, such as Cl−. Fromell’s study [15], the binding ability of Cl− is stronger than that ofO3

−, so the residual NO3− can be also removed by Cl− and the

ig. 3. SERS spectra of hydroxylamine-reduced silver colloid when adding differentoncentrations of NaNO3.

Fig. 4. SERS spectra of hydroxylamine-reducedsilver colloid when adding differentconcentrations of NaCl.

band at 239 cm−1 is assigned to the strong Ag Cl bonds. The addi-tion of Cl− leads at least two effects to the colloid: the aggregation ofthe colloid and the modification of the surface chemistry of the sil-ver particles. The aggregation properties of silver nanoparticles aremostly influenced by their surface charge properties [19,24]. At thelow concentration of Cl−, residual impurities binding on the silversurface is replaced partially by Cl− and the silver colloid is aggre-gated to some extent. And then, the Raman spectrum of residualimpurities which had not been replaced by Cl− is observed. Withthe increase of Cl− concentration, residual impurities are replacedgradually and the residual impurity features are replaced by thestrong Ag Cl band.

Fig. 5 shows a series of Raman spectra of HRSC to which increas-ing amounts of Na2SO4 was added. The results are quite differentfrom Figs. 3 and 4. In this work, it is found that the three peaks at239 cm−1 which are assigned to Ag-anions bonds are superposedfor the three different concentration of Na2SO4 (0.02 mol dm−1,0.04 mol dm−1, 0.06 mol dm−1). However, the intensity of residualimpurity bands is increased to a certain extent with the increaseof Na2SO4. The bands at 1178, 1362, 1508, 1580 cm−1 which areassigned to impurity bands are observed clearly even though the

Na2SO4 is added at a high concentration (0.06 mol dm−1). Thesuperposition of bands at 239 cm−1 implies that the addition ofSO4

2− cannot change the amount of Ag-anions bonds. That means

Fig. 5. SERS spectra of hydroxylamine-reduced silver colloid when adding differentconcentrations of Na2SO4.

100 X. Dong et al. / Spectrochimica Acta Part A 88 (2012) 97– 101

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Fig. 6. The comparison of affinity among Cl− , NO3− , and I− .

he band at 239 cm−1 can be assigned to the bonds between Agnd residual chloride ions which are adsorbed on Ag particles, andhe binding ability of Cl− is stronger than that of SO4

2− and theesidual Cl− cannot be removed by SO4

2−. The increased bands at178, 1362, 1508, 1580 cm−1 means that the residual impuritiesannot be replaced by SO4

2−. The enhancement of residual impu-ity bands is arising from simple aggregation of the colloid. Fromhe above, the affinity order among the three kinds of anions is:l− > NO3

− > SO42−. Fig. 6 compares the affinities of Cl−, NO3

−, and−. From Fig. 6 it is observed that addition of I− does not cause anybvious change of spectrum of HRSC. The reason is that the affinityf I− is so strong that it replaces all of the residual impurities andesidual nitrate ions on the silver particles. The intense bands ofg Cl at 239 cm−1 are removed by addition of I−.

The affinity ability of impurities is stronger than the affinitybility of SO4

2−, but it is weaker than that of strongly bindingnions such as NO3

−, Cl−, and I−. The strongly binding ions Cl−

an formed the intense Ag Cl band. Hydroxylamine-reduced sil-er colloid is not normally used to enhance the Raman spectra ofesidual ions, but it can be used to give SERS spectra of residual ionsnder appropriate conditions. So when attempting to detect SERSignals of some analytes, it may face the problem that it cannot beure whether the Raman bands is the bands of analyte or the bandsf residual ions. For this reason it is very important to give a com-arative analysis among the SERS signal of analyte, residual ions inolloid and the analyte at a higher concentration.

Fig. 7a shows the SERS spectra of MB when adding NaCl, NaNO3,a2SO4, and NaI to the silver colloid. Through comparison of the five

pectra, it has been noted that extra adding of anions can enhancehe SERS signal of MB to a certain extent. The SERS activation thats induced by anions can be attributed to the increased electronicnteraction between Ag and MB molecules. It can be also observedlearly that Cl− gives the greatest enhancement among differentinds of anions, while I−, the most strongly binding anion, giveshe least enhancement for SERS. The enhancement factor of SERSan be calculated from the formulation [12,25,26]

= IAg/Nads

Isol/Nsol

here IAg and Isol are the intensities of the same band for MB inhe SERS spectrum and in the normal Raman spectrum, respec-

ively. Nads is the number of MB molecules in the colloid in laserctivation volume, and the Nsol is the number of MB moleculesn aqueous solution in laser activation volume. Nads and Nsol areirectly proportional to the concentration of MB in colloid and in

Fig. 7. (a) SERS spectra of MB with hydroxylamine-reduced silver colloid when NaCl,NaNO3, Na2SO4, and NaI were added respectively. (b) Normal Raman spectrum of100 mM MB aqueous solution.

aqueous solution, respectively. From the calculation, the enhance-ment factors for MB with Cl−, NO3

−, SO4−2 and I− are 2.3 × 105,

1.55 × 105, 7.35 × 104, 4.1 × 104, respectively (we take the band at1626 cm−1 as the object to calculate the enhancement factors). Theenhancement factor is 2.77 × 104 for MB without any aggregatingagents. From previous studies [27,28], the aggregation of silver par-ticles (electromagnetic mechanism) plays a key role in SERS whilethe modification of silver surface chemistry (charge transfer mech-anism) only enhance the SERS signal a few fold. The addition ofstrongly binding anions mainly causes the modification of silversurface chemistry, and the modification of silver surface by stronglybinding anions changes the binding affinity of MB and it lead toaggregation of silver particles. Because the Ag surface which hasbeen modified by extra anions has negative charge while the MBmolecules have positive charge, the MB molecules can adsorb on Agsurface easily. It can be attributed to the cooperative effects of theelectromagnetic mechanism and the charge transfer mechanism.The electromagnetic mechanism is due to an electromagnetic fieldenhancement caused by a plasmon excitation through the incidentlaser beam on the silver surface [16]. The electrons in the silverare excited to an oscillation against the silver cores, which is calledsurface plasmons. The excited surface plasmons lead to an elec-tromagnetic field, and when the MB molecules are located at theelectromagnetic field the SERS signal is greatly enhanced. Whenadding anions to the colloid, the silver particles aggregated andyield “hot spots” near the touching points of the particles. At the

“hot spots” the electromagnetic field is especially concentrated andyield higher enhancement than the non-aggregated particles. Inthat case, there is a strong chemical enhancement acting in con-cert with the electromagnetic effect which is called chemical effect

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r charge transfer mechanism. The charge transfer mechanism isssociated with a charge transfer process between the adsorbedB molecules and the silver surface. As for the weakly binding

nions, such as SO42−, the effect of extra anions is aggregate the

ilver particles only. Fig. 7b shows the normal Raman spectrum of00 mM MB aqueous solution. The bands are observed at 449, 499,02, 673, 776, 812, 862, 908, 952, 1038, 1072, 1161, 1301, 1394,474, 1627 cm−1. The band at 239 cm−1 which is assigned to Ag-nions bonds is also observed when adding Cl−, NO3

−, and SO42−

s aggregating agents, and the band is increased in proportion toB bands. It means the addition of MB molecules cannot replace

esidual Cl− on silver particles.However, many bands in the SERS spectrum have a small shift

n position relative to their corresponding normal Raman spec-rum. For example, the bands at 449, 499 cm−1 which have beenssigned to C N C skeletal bending [25,29] in normal Raman spec-rum shift to 452, 486 cm−1 in SERS spectra, respectively. Theseands at 1394 and 1627 cm−1 which have been assigned to C Ntretching and C C stretching in normal Raman spectrum shift to391, 1624 cm−1 in SERS spectra. In comparison with the normalaman spectrum, some bands split in the SERS spectra. For instance,he band at 1474 cm−1 in Fig. 7b splits into two peaks at 1442 and478 cm−1. The shifts and splits of these Raman bands indicate thatB molecules may be chemisorbed on the silver nanoparticles sur-

ace and chemical effects are the main mechanism responsible forhe relative shifts [25,30].

. Conclusions

The results show that impurities can affect the SERS spectrum ofRSC, but this inconvenience can be overcame by the identificationf impurities bands and the modification of silver particles sur-ace chemistry. When adding some extra anions to the HRSC somentense bands appear, and these bands are assigned to the bands ofesidual ions or the band of Ag-anions. Cl− on the silver particles canorm the anomalously intense Ag Cl band. The affinity order amonghese kinds of anions is: I− > Cl− > NO3

− > SO42−. From detecting the

ERS spectroscopy of MB, the function of strongly binding anions isainly to modify the silver particles surface chemistry and change

he binding affinity of MB which leading to the aggregation of sil-er particles. Among the four kinds of anions, Cl− gives the greatestnhancement, and it is due to the cooperative effects of the mod-fication of silver surface chemistry and the aggregation of silverarticles. As for the weakly binding anions, such as SO4

2−, the effectf extra anions is to aggregate the silver particles. The purpose ofhis paper is to further investigate the effect of anions on HRSCnd the Raman spectroscopy of residual ions in HRSC, and makeshe experimental conditions suitable for detecting some analytesn high efficiency.

cknowledgement

This research is supported by the National Natural Science Foun-ation of China (60678050).

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