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Journal of EnvironmentalScience and Health .Part A: EnvironmentalScience and Engineeringand ToxicologyPublication details, including instructions forauthors and subscription information:http://www.tandfonline.com/loi/lesa19

Influence of chlorideion on sers of humicsubstances adsorbed onsilver solsYu‐hui Yang a b , Qun Zhou a & Jian‐yuan Yu a

a Chinese Academy of Sciences , Institute ofChemistry , Beijing, 100080, Chinab Analyical and testing centre , IsinghuaUniversity , Beijing, 100086, ChinaPublished online: 15 Dec 2008.

To cite this article: Yu‐hui Yang , Qun Zhou & Jian‐yuan Yu (1996)Influence of chloride ion on sers of humic substances adsorbed onsilver sols, Journal of Environmental Science and Health . Part A:Environmental Science and Engineering and Toxicology, 31:6, 1395-1405, DOI:10.1080/10934529609376431

To link to this article: http://dx.doi.org/10.1080/10934529609376431

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J. ENVIRON. SCI. HEALTH, A31(6), 1395-1405 (1996)

INFLUENCE OF CHLORIDE ION ON SERS OF HUMICSUBSTANCES ADSORBED ON SILVER SOLS

Key words: humic acid ( H A ) , fulvic acid ( F A ) , Surface-enhancedRaman Scattering (SERS), chloride ion

Yu-hui Yang*(Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China)

Qun Zhou and Jian-yuan Yu(Analyical and testing centre, Isinghua University, Beijing 100086, China)

ABSTRACT

The SER spectra of humic substances in neutral pH and low concentrations

typically found in natural environment could be conveniently observed while

a laser-reduced silver colloid made from AgNO3 was used. However, their SER

signals were weaken and then scavenged as a small amount of KC1 was

progressively added. Therefore, it was suggested that a desalted procedure was

helpful for trace detection of humic substances in real environment by SER

spectroscopy while silver sols were used for the substrates.

INTRODUCTION

In order to trace detect and characterize humic substances andco-existed organic species in natural environment, it is essential to

* To whom correspondence should be addressed. Present address: WageningenAgricultural University, Department of Physical and Colloid Chemistry,6703 HB Wageningen, the Netherlands. Email: Yang@Fenk.WAU.NL

1395

Copyright © 1996 by Marcel Dekker, Inc.

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1396 YANG, ZHOU, AND YU

develop some non - destructive, non - separative, high sensitive andhigh selective approaches. Surface - enhanced Raman spectroscopy1

had been proved to be a very promising tool which meets thesituations above2"3. When chemical-reduced silver sols are used forthe SER substrates, at neutral and basic conditions, SER signals of co-existed organic compounds are further enhanced due to humus -induced resonance effect without spectral interferences fromhumus3; on the other hand, the SERR signals of humic substances arepredominantly observed at acidic conditions2"3. This method isindeed very good for the SERS observation of many organiccompounds in the presence of humic substances. However, the SERRsignals of humic substances are rather poor reproducible due to theinstability of colloidal silvers at acidic conditions. Besides, it is moreinteresting to directly observe SERS characteristics of humic andfulvic acid solutions at neutral and weakly basic conditions typicallyfound in real environment. Thus, in the present study, we first try touse a simple laser-reduced silver sol4 as the SERS substrate.

Chloride ion extensively exist in natural environment and itsconcentrations are generally much higher than those of humus andother organic species5-6. On the other hand, chloride ion has also agreat influence on the SERS effect of many organic compounds7-8.Clearly, in order to develop SER spectroscopy as a truly powerful toolfor trace analysis and characterization of humus and other organicspecies in real environment, it is essential to clarify the influence ofchloride ion on their SERS characteristics. Therefore, in this paper, apreliminary study to this problem pertinent to humus is addressed.

EXPERIMENTAL

The fulvic and humic acids from various sources2 were dissolvedin triply-distilled water by adding freshly prepared NaoH solutions.The concentrations of the stock solutions were ca. 100 mg/L and thepHs were ca. 9. All the chemical reagents used here were A.R. grade.The mixtures consisted of a humic sample and a organic compound3

as well as a water sample from a stream with yellow colour kindlyprovided by the Insititue of Ganlan Meadow Ecology ( Lanzhou, P.R.China ) were also used for analysis.

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CHLORIDE ION AND SERS OF HUMIC SUBSTANCES 1397

After filtered through a 0.45 um membrane, 50 ml of the water

sample was first led through a Novapak Ci8 column ( 150 x 4 mm )

and then the column was washed by water ( Mobile A ) and 60 %

methanol ( Mobile B ), respectively. The organic species in the sample

were separated into the hydrophilic and hydrophobic fractions

according to the procedure. The HPLC system consisted of a Beckman

Model 110 A solvent deliery and a Beckman Model 160 ultraviolet

detector operated at 254 nm. The collected fractions were diluted to

50 ml for analysis.

In a typical case, 40 ul of each test sample was added to 4.0 ml

1.0 x 10-3 M AgNO3 solution and the pH value could be adjusted by

using refined strong acidic ion-exchange resin ( H + and Na+ form ).

The resulting solution was vortex - mixed to give complete

homogenization. ca. 1 ml of the solution was put into a conventional 1

x 1 cm2 quartz cell for Raman measurement.

Different volume of 1 M KC1 solutions were also added to the

AgNO3-humus systems above and the systems were illuminated for

Raman analysis.

In some parellel experiments, the chemical-reduced yellow silver

so l 9 was used instead of the laser-reduced silver sol4.

All the spectra were recorded on a Spex Model 1403 double

monochromator equipped with a RCA C-31034 photomultiplier tube.

Laser excitation at 514.5 nm was provided by a Spectra - physics Ar+

laser ( Model 2020 - 5 ). Data acqusition conditions: Integration time

1 s; Scan step length: 2 cm'1 ; Slit width: 400 um; Scan times: 2; Laser

power: 30 mW; Five - data - point smoothed.

RESULTS AND DISCUSSION

In the presence of organic compounds, Ag+ is in situ photoreduced

into Ag and forms very good SER substrates4. The AgNO3 solutions in

the presence of humic or fulvic acids indeed produced very strong

SERS effect under Ar+ laser illumination. Good-quality SER spectra of

every humic sample in neutral and weakly basic conditions could be

rapidly obtained. It implies that humus is simultaneously adsorbed

on the silver surface as the silvers which bind with it are reduced

under laser illumination. Several representative results are shown in

Figure 1.

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1398 YANG, ZHOU, AND YU

1075Raman Shift ( cnr1 )

2000

Fig.l. SER spectra of AgNO3 photocoUoid ( 1 x 10"3 M )A. blank, pH 9B. 1 mg/L of the fulvic acid from sea water, pH 9C. 1 mg/L of the humic acid from sea water, pH 6D. 15 mg/L of the fulvic acid from peat, the concentration of AgNOß is

2 x 10"3 M, pH 9.

In almost all the spectra, there are only two strong and broadbands centred at ca. 1350 or 1380 cnr1 and 1590 - 1620 cm"1. Thebands are certainly assigned to the D band and G band, respectively,of disordered carbons10. Since the disordered carbon networks arethe inherent " backbone " characteristics of humic substances'1, it isclear that the Raman11-12 and SER spectra2 of humic substancesmainly reveal their " backbones ". Therefore, it is also well-

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CHLORIDE ION AND SERS OF HUMIC SUBSTANCES 1399

understood why their Raman and SER spectra are insensitive to thechanges of pH values ( e.g., comparing Fig.l here with Fig.3 of ref.2 ).

Although the similar results had been obtained under acidicconditions by using chemical - reduced silver sols2, the photocolloidshows several advantages over the chemical - reduced colloids forthe SERS detection of humic substances. First, when chemical-reduced sols are used, only under very acidic conditions ( ca. pHl-3 ),the SERS signals of humic substances can be obviously observed.However, the sols are unstable and rapidly precipitate under theconditions, thus the results are poor reproducible in some cases.When the photocolloid is used, humic substances at neutral andweakly basic conditions typically occured in natural environment canbe reliably detected by SER spectroscopy. Second, due to their limitedabsorption capacities, chemical - reduced sols can be only used whenthe concentrations of humic substances in the test systems are lowerthan 4 - 10 mg/L, unless the fluorescences from humic substanceswill predominate in the spectra. However, the concentration of thephotocolloid can be easily adjusted to solve this problem ( e.g., seeFig. ID ). Besides, the photocolloid spectra are rather better in termsof signal-to-noise ratio and its sensitivity can be further enhanced byincreasing the concentration of AgNÛ3 solution4.

To some mixtures consist of a humic or fulvic acid and a organiccompound 3 , it is generally found that the broad and strong bands ofgraphite-like structures from humic substance predominate in thespectra although the SER signals of the compounds are also observedin some cases. A typical result is illustrated in Figure 2.

Fig.2A is the SER spectrum of pyridine. The signal-to-noise of thespectrum is rather low and there is also a obvious backgroundscattering between 1700 and 1100 cm"1 which is attributable tographite carbon on the surface due to the decomposition of pyridine4.In the presence of the humic acid, the SER signals of pyridine arefurther enhanced ( Fig.2A ). It is possibly from humus - inducedresonance effect3. Besides, the appearance of the bands at 1212 and1596 cm"1 and the changes of the relative intensity ( Iioos/11036 )shows the " flat on " configuration of pyridine on the surface. That isto say, a change of orientation of adsorbed pyridine occurs in the

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1400 YANG, ZHOU, AND YU

300

150 1075

Raman shift ( cm"1 )

2000

Fig.2. SER spectra of AgNO3 photocolloid ( 3.4 x 10"4 M )A. 10'3 M of pyridineB. 10'3 M of pyridine and 1 mg/L of the fulvic acid from weathered coal

of Gongxian.

presence of humic substances3. At the time, the two broad and strongbands between 1100 cm"' and 1700 cm'1 are only from the humicacid3 rather than the artifact4.

Based on the results above, we prefer to use the photocolloid4 fortrace detection of humic substances in complex systems and evenreal environments. On the other hand, chemical-reduced sols shouldbe better for analysis and characterization of organic species in thepresence of humic substances3.

When a small of 1 M KC1 is added to the systems consist of AgNC>3and humic substances, the SERS signals of humic substances aregenerally weaken and then scavenged as the concentrations of KC1are progressively increased. A typical example is shown in Figure 3.

In Fig.3, the band at 240 cm-' is assigned to the formation of Ag-Cl bond8. It is generally found that the SER signals of humicsubstances become much weaker while the concentrations of chlorideion are ca. 30 - 50 % of the concentrations of AgNO3. However, much

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CHLORIDE ION AND SERS OF HUMIC SUBSTANCES 1401

1075

Raman shift ( cm"1 )

2000

Fig.3. SER spectra of Shitara Black HA ( I mg/L ) in the use of AgNO3photocolloid ( 1 x I0"3 M ).A. no Cl" ionB. 3.4 x 10-4 M of Cl" ionC. 5.0 x 10-4 M of Cl" ionD. 1.0 x 10"3 M of CI" ionE. 2.0 x 10-3 M of CI" ion.

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1402 YANG, ZHOU, AND YU

Time ( min ) 1.0 5 0

Fig.4. RP-HPLC chromatogram. of the desalted water sample

more chloride ions are usually necessary to fully scavenge the SERsignals of humic substances. Similar results are also observed whenthe yellow silver sol is used ( not shown ).

This phenomenon is reasonable because the stability constants ofchloride complexes are much higher than those of many organicspecies in natural environment for silver5-6. Thus humic substancescan not compete with chloride ion for the active sites of the surfacesand their SER spectra are only observed while the co-existed Cl" ionsdo not cover all the " active sites " of the surfaces.

Clearly, this phenomenon is disadvangeous to in situ detect humicsubstances in real environment. In order to solve the problem, adesalted procedure should be helpful for trace detect of humic and

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CHLORIDE ION AND SERS OF HUMIC SUBSTANCES 1403

1376 1590

1075

Raman shift (cm-1 )2000

Fig.5. SER spectra after mixing 0.5 ml of the sample with 0.5 ml 2 x 10"3 MAgNO3

A. the water sampleB. its hydrophilic fractionC. its hydrophobic fraction.

fulvic acids in complex systems as well as real environment whenthe concentrations of Cl" and some other inorganic ions such as Br-and I" are high. According to this assumption , the dissolved humusin a water sample from real environment was first desalted byconventional RP-HPLC methodology13 and then successfully detectedby SER spectroscopy. The results are shown in Figures 4 and 5.

Fig.5A is the SER spectrum of the original water sample. Clearly,this spectrum is poor and the humus in the sample can not reliablydetected. The strong band at 240 cm-' is possibly from chloride ions

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1404 YANG, ZHOU, AND YU

in the sample. When the water sample was led through a RP-HPLC

column and inorganic species were first removed, the organic species

were effectively separated into the hydrophilic and hydrophobic

f r a c t i o n s 1 3 ( Fig.4B ). After dilution, although the concentrations of

the organic species are lower than that of the original sample, their

SER signals are enough strong and undoubtedly reveal the presence

of humus in the water ( Fig. 5B and 5C ). The procedure supports our

assumption above. By the way, it also implies that the fractionational

p r o c e d u r e 1 3 can not effectively disrupt the backbone of the humic

sample since both of the hydrophilic and hydrophobic fractions

behave as graphite-like carbons ( Fig.5B and 5C ).

CONCLUSIONS

The laser-reduced silver colloid made from AgNO3 is a excellent

substrate for trace detection of humic substances in natural pH and

concentrations by SER spectroscopy. However, the co-existed Cl" ion

has a negative effect on their SERS observation and thus it should be

removed prior to the detection in some cases.

ACKNOWLEDGEMENTS

The authors would like to express their deep gratitude to Prof. S.

Kuwatsuka et al. ( Faculty of Agriculture, Nagoya University, Japan )

for providing soil humic acids and to Prof. Hayano and Dr. Y. Sakurai

( Institute of Industrial Science, University of Tokyo, Japan ) for

providing fulvic and humic acids from marine sediments.

REFERENCES

1. R.K. Chang and T.E. Furtak (Eds.), Surface Enhanced Raman Scatter-ing, New York, Plenum Press, 1982.

2. Y. Yang and D. Zhang, Surface-enhanced Raman Scattering of HumicSubstances Adsorbed on Silver Sols, Spectrosc. Lett., 1995, 28: 1203and some references cited herein.

3. Y. Yang and D. Zhang, Trace Detection and Characterization of Humic

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CHLORIDE ION AND SERS OF HUMIC SUBSTANCES 1405

Substances and Co-existed Organic Species, Toxic. Environ. Chem., inpress.

4. A.M. Ahern and R.L. Garrell, In Situ Photoreduced Silver Nitrate asa Substrate for Surface-enhanced Raman Spectroscopy, Anal. Chem.1987, 59: 2816.

5. J. Buffle, Complexation Reactions in Aquatic Systems: An AnalyticalApproach, Ellis Horwood, Chichester, 1988.

6. L.A. Miller and K.W. Bruland, Organic Speciation of Silver in MarineWaters, Environ. Sci. Technol., 1995, 29: 2616.

7. P. Hildeberandt and M. Stockburger, Surface-enhanced ResonanceRaman Spectroscopy of Rhodamine 6G Adsorbed on ColloidalSilver, J. Phys. Chem., 1984, 88: 5935.

8. S.Y. Fu and P.X. Zhang, Chemical Effect of Chloride Ions on SERS inSilver Sol, J. Raman Spectrosc., 1992, 23: 93.

9. J.J. Laserna, E.L. Torres and J.D. Winefordner, Studies of SamplePreparation for Surface-enhanced Raman Spectroscopy on SilverHydrosols, Anal. Chim. Acta, 1987, 200: 469.

10. F. Tuinstra and J.L. Koening, Raman Spectra of Graphite, J. Chem.Phys., 1970, 53: 1126.

11.Y. Yang and T. Wang, Fourier Transform Raman SpectroscopicCharacterization of Humic Substances, Carbon, in press.

12. Y. Yang, B. Li and Z. Tao, Characterization of Humic Substances byLaser Raman Spectroscopy, Spectrosc. Lett., 1994, 27: 649.

13. F.Y. Saleh, W.A. Ong and D.Y. Chang, Structural Features of AquaticFulvic Acids: Analytical and Preparative Reverse-Phase High-Performance Liquid Chromatography Separation with Photodiode

Array Detection, Anal. Chem., 1989, 61: 2792.

Received: February 21, 1996Accepted: March 26, 1996

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