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Sensors & TransducersVolume 85Issue 11November 2007www.sensorsportal.comISSN 1726-5479Editor-in-Chief: professor Sergey Y. Yurish, phone: 34 696067716, fax: 34 93 4011989,e-mail: [email protected] for Western EuropeMeijer, Gerard C.M., Delft University of Technology, The NetherlandsFerrari, Vitorio, Universitá di Brescia, ItalyEditors for North AmericaDatskos, Panos G., Oak Ridge National Laboratory, USAFabien, J. Josse, Marquette University, USAKatz, Evgeny, Clarkson University, USAEditor South AmericaCosta-Felix, Rodrigo, Inmetro, BrazilEditor for Eastern EuropeSachenko, Anatoly, Ternopil State Economic University, UkraineEditor for AsiaOhyama, Shinji, Tokyo Institute of Technology, JapanEditorial Advisory BoardAbdul Rahim, Ruzairi, Universiti Teknologi, MalaysiaAhmad, Mohd Noor, Nothern University of Engineering, MalaysiaAnnamalai, Karthigeyan, National Institute of Advanced IndustrialScience and Technology, JapanArcega, Francisco, University of Zaragoza, SpainArguel, Philippe, CNRS, FranceAhn, Jae-Pyoung, Korea Institute of Science and Technology, KoreaArndt, Michael, Robert Bosch GmbH, GermanyAscoli, Giorgio, George Mason University, USAAtalay, Selcuk, Inonu University, TurkeyAtghiaee, Ahmad, University of Tehran, IranAugutis, Vygantas, Kaunas University of Technology, LithuaniaAvachit, Patil Lalchand, North Maharashtra University, IndiaAyesh, Aladdin, De Montfort University, UKBahreyni, Behraad, University of Manitoba, CanadaBaoxian, Ye, Zhengzhou University, ChinaBarford, Lee, Agilent Laboratories, USABarlingay, Ravindra, Priyadarshini College of Engineering andArchitecture, IndiaBasu, Sukumar, Jadavpur University, IndiaBeck, Stephen, University of Sheffield, UKBen Bouzid, Sihem, Institut National de Recherche Scientifique, TunisiaBinnie, T. David, Napier University, UKBischoff, Gerlinde, Inst. Analytical Chemistry, GermanyBodas, Dhananjay, IMTEK, GermanyBorges Carval, Nuno, Universidade de Aveiro, PortugalBousbia-Salah, Mounir, University of Annaba, AlgeriaBouvet, Marcel, CNRS – UPMC, FranceBrudzewski, Kazimierz, Warsaw University of Technology, PolandCai, Chenxin, Nanjing Normal University, ChinaCai, Qingyun, Hunan University, ChinaCampanella, Luigi, University La Sapienza, ItalyCarvalho, Vitor, Minho University, PortugalCecelja, Franjo, Brunel University, London, UKCerda Belmonte, Judith, Imperial College London, UKChakrabarty, Chandan Kumar, Universiti Tenaga Nasional, MalaysiaChakravorty, Dipankar, Association for the Cultivation of Science, IndiaChanghai, Ru, Harbin Engineering University, ChinaChaudhari, Gajanan, Shri Shivaji Science College, IndiaChen, Rongshun, National Tsing Hua University, TaiwanCheng, Kuo-Sheng, National Cheng Kung University, TaiwanChiriac, Horia, National Institute of Research and Development, RomaniaChowdhuri, Arijit, University of Delhi, IndiaChung, Wen-Yaw, Chung Yuan Christian University, TaiwanCorres, Jesus, Universidad Publica de Navarra, SpainCortes, Camilo A., Universidad de La Salle, ColombiaCourtois, Christian, Universite de Valenciennes, FranceCusano, Andrea, University of Sannio, ItalyD'Amico, Arnaldo, Università di Tor Vergata, ItalyDe Stefano, Luca, Institute for Microelectronics and Microsystem, ItalyDeshmukh, Kiran, Shri Shivaji Mahavidyalaya, Barshi, IndiaKang, Moonho, Sunmoon University, Korea SouthKaniusas, Eugenijus, Vienna University of Technology, AustriaKatake, Anup, Texas A&M University, USADickert, Franz L., Vienna University, AustriaDieguez, Angel, University of Barcelona, SpainDimitropoulos, Panos, University of Thessaly, GreeceDing Jian, Ning, Jiangsu University, ChinaDjordjevich, Alexandar, City University of Hong Kong, Hong KongDonato, Nicola, University of Messina, ItalyDonato, Patricio, Universidad de Mar del Plata, ArgentinaDong, Feng, Tianjin University, ChinaDrljaca, Predrag, Instersema Sensoric SA, SwitzerlandDubey, Venketesh, Bournemouth University, UKEnderle, Stefan, University of Ulm and KTB mechatronics GmbH,GermanyErdem, Gursan K. Arzum, Ege University, TurkeyErkmen, Aydan M., Middle East Technical University, TurkeyEstelle, Patrice, Insa Rennes, FranceEstrada, Horacio, University of North Carolina, USAFaiz, Adil, INSA Lyon, FranceFericean, Sorin, Balluff GmbH, GermanyFernandes, Joana M., University of Porto, PortugalFrancioso, Luca, CNR-IMM Institute for Microelectronics andMicrosystems, ItalyFu, Weiling, South-Western Hospital, Chongqing, ChinaGaura, Elena, Coventry University, UKGeng, Yanfeng, China University of Petroleum, ChinaGole, James, Georgia Institute of Technology, USAGong, Hao, National University of Singapore, SingaporeGonzalez de la Ros, Juan Jose, University of Cadiz, SpainGranel, Annette, Goteborg University, SwedenGraff, Mason, The University of Texas at Arlington, USAGuan, Shan, Eastman Kodak, USAGuillet, Bruno, University of Caen, FranceGuo, Zhen, New Jersey Institute of Technology, USAGupta, Narendra Kumar, Napier University, UKHadjiloucas, Sillas, The University of Reading, UKHashsham, Syed, Michigan State University, USAHernandez, Alvaro, University of Alcala, SpainHernandez, Wilmar, Universidad Politecnica de Madrid, SpainHomentcovschi, Dorel, SUNY Binghamton, USAHorstman, Tom, U.S. Automation Group, LLC, USAHsiai, Tzung (John), University of Southern California, USAHuang, Jeng-Sheng, Chung Yuan Christian University, TaiwanHuang, Star, National Tsing Hua University, TaiwanHuang, Wei, PSG Design Center, USAHui, David, University of New Orleans, USAJaffrezic-Renault, Nicole, Ecole Centrale de Lyon, FranceJaime Calvo-Galleg, Jaime, Universidad de Salamanca, SpainJames, Daniel, Griffith University, AustraliaJanting, Jakob, DELTA Danish Electronics, DenmarkJiang, Liudi, University of Southampton, UKJiao, Zheng, Shanghai University, ChinaJohn, Joachim, IMEC, BelgiumKalach, Andrew, Voronezh Institute of Ministry of Interior, RussiaRodriguez, Angel, Universidad Politecnica de Cataluna, SpainRothberg, Steve, Loughborough University, UK

Kausel, Wilfried, University of Music, Vienna, AustriaKavasoglu, Nese, Mugla University, TurkeyKe, Cathy, Tyndall National Institute, IrelandKhan, Asif, Aligarh Muslim University, Aligarh, IndiaKim, Min Young, Koh Young Technology, Inc., Korea SouthKo, Sang Choon, Electronics and Telecommunications Research Institute,Korea SouthKockar, Hakan, Balikesir University, TurkeyKotulska, Malgorzata, Wroclaw University of Technology, PolandKratz, Henrik, Uppsala University, SwedenKumar, Arun, University of South Florida, USAKumar, Subodh, National Physical Laboratory, IndiaKung, Chih-Hsien, Chang-Jung Christian University, TaiwanLacnjevac, Caslav, University of Belgrade, SerbiaLaurent, Francis, IMEC , BelgiumLay-Ekuakille, Aime, University of Lecce, ItalyLee, Jang Myung, Pusan National University, Korea SouthLee, Jun Su, Amkor Technology, Inc. South KoreaLei, Hua, National Starch and Chemical Company, USALi, Genxi, Nanjing University, ChinaLi, Hui, Shanghai Jiaotong University, ChinaLi, Xian-Fang, Central South University, ChinaLiang, Yuanchang, University of Washington, USALiawruangrath, Saisunee, Chiang Mai University, ThailandLiew, Kim Meow, City University of Hong Kong, Hong KongLin, Hermann, National Kaohsiung University, TaiwanLin, Paul, Cleveland State University, USALinderholm, Pontus, EPFL - Microsystems Laboratory, SwitzerlandLiu, Aihua, Michigan State University, USALiu Changgeng, Louisiana State University, USALiu, Cheng-Hsien, National Tsing Hua University, TaiwanLiu, Songqin, Southeast University, ChinaLodeiro, Carlos, Universidade NOVA de Lisboa, PortugalLorenzo, Maria Encarnacio, Universidad Autonoma de Madrid, SpainLukaszewicz, Jerzy Pawel, Nicholas Copernicus University, PolandMa, Zhanfang, Northeast Normal University, ChinaMajstorovic, Vidosav, University of Belgrade, SerbiaMarquez, Alfredo, Centro de Investigacion en Materiales Avanzados,MexicoMatay, Ladislav, Slovak Academy of Sciences, SlovakiaMathur, Prafull, National Physical Laboratory, IndiaMaurya, D.K., Institute of Materials Research and Engineering, SingaporeMekid, Samir, University of Manchester, UKMendes, Paulo, University of Minho, PortugalMennell, Julie, Northumbria University, UKMi, Bin, Boston Scientific Corporation, USAMinas, Graca, University of Minho, PortugalMoghavvemi, Mahmoud, University of Malaya, MalaysiaMohammadi, Mohammad-Reza, University of Cambridge, UKMolina Flores, Esteban, Benemirita Universidad Autonoma de Puebla,MexicoMoradi, Majid, University of Kerman, IranMorello, Rosario, DIMET, University "Mediterranea" of Reggio Calabria,ItalyMounir, Ben Ali, University of Sousse, TunisiaMukhopadhyay, Subhas, Massey University, New ZealandNeelamegam, Periasamy, Sastra Deemed University, IndiaNeshkova, Milka, Bulgarian Academy of Sciences, BulgariaOberhammer, Joachim, Royal Institute of Technology, SwedenOuld Lahoucin, University of Guelma, AlgeriaPamidighanta, Sayanu, Bharat Electronics Limited (BEL), IndiaPan, Jisheng, Institute of Materials Research & Engineering, SingaporePark, Joon-Shik, Korea Electronics Technology Institute, Korea SouthPereira, Jose Miguel, Instituto Politecnico de Setebal, PortugalPetsev, Dimiter, University of New Mexico, USAPogacnik, Lea, University of Ljubljana, SloveniaPost, Michael, National Research Council, CanadaPrance, Robert, University of Sussex, UKPrasad, Ambika, Gulbarga University, IndiaPrateepasen, Asa, Kingmoungut's University of Technology, ThailandPullini, Daniele, Centro Ricerche FIAT, ItalyPumera, Martin, National Institute for Materials Science, JapanRadhakrishnan, S. National Chemical Laboratory, Pune, IndiaRajanna, K., Indian Institute of Science, IndiaRamadan, Qasem, Institute of Microelectronics, SingaporeRao, Basuthkar, Tata Inst. of Fundamental Research, IndiaReig, Candid, University of Valencia, SpainRestivo, Maria Teresa, University of Porto, PortugalRezazadeh, Ghader, Urmia University, IranRobert, Michel, University Henri Poincare, FranceRoyo, Santiago, Universitat Politecnica de Catalunya, SpainSadana, Ajit, University of Mississippi, USASandacci, Serghei, Sensor Technology Ltd., UKSapozhnikova, Ksenia, D.I.Mendeleyev Institute for Metrology, RussiaSaxena, Vibha, Bhbha Atomic Research Centre, Mumbai, IndiaSchneider, John K., Ultra-Scan Corporation, USASeif, Selemani, Alabama A & M University, USASeifter, Achim, Los Alamos National Laboratory, USASengupta, Deepak, Advance Bio-Photonics, IndiaShearwood, Christopher, Nanyang Technological University, SingaporeShin, Kyuho, Samsung Advanced Institute of Technology, KoreaShmaliy, Yuriy, Kharkiv National University of Radio Electronics,UkraineSilva Girao, Pedro, Technical University of Lisbon PortugalSlomovitz, Daniel, UTE, UruguaySmith, Martin, Open University, UKSoleymanpour, Ahmad, Damghan Basic Science University, IranSomani, Prakash R., Centre for Materials for Electronics Technology,IndiaSrinivas, Talabattula, Indian Institute of Science, Bangalore, IndiaSrivastava, Arvind K., Northwestern UniversityStefan-van Staden, Raluca-Ioana, University of Pretoria, South AfricaSumriddetchka, Sarun, National Electronics and Computer TechnologyCenter, ThailandSun, Chengliang, Polytechnic University, Hong-KongSun, Dongming, Jilin University, ChinaSun, Junhua, Beijing University of Aeronautics and Astronautics, ChinaSun, Zhiqiang, Central South University, ChinaSuri, C. Raman, Institute of Microbial Technology, IndiaSysoev, Victor, Saratov State Technical University, RussiaSzewczyk, Roman, Industrial Research Institute for Automation andMeasurement, PolandTan, Ooi Kiang, Nanyang Technological University, Singapore,Tang, Dianping, Southwest University, ChinaTang, Jaw-Luen, National Chung Cheng University, TaiwanThumbavanam Pad, Kartik, Carnegie Mellon University, USATsiantos, Vassilios, Technological Educational Institute of Kaval, GreeceTsigara, Anna, National Hellenic Research Foundation, GreeceTwomey, Karen, University College Cork, IrelandValente, Antonio, University, Vila Real, - U.T.A.D., PortugalVaseashta, Ashok, Marshall University, USAVazques, Carmen, Carlos III University in Madrid, SpainVieira, Manuela, Instituto Superior de Engenharia de Lisboa, PortugalVigna, Benedetto, STMicroelectronics, ItalyVrba, Radimir, Brno University of Technology, Czech RepublicWandelt, Barbara, Technical University of Lodz, PolandWang, Jiangping, Xi'an Shiyou University, ChinaWang, Kedong, Beihang University, ChinaWang, Liang, Advanced Micro Devices, USAWang, Mi, University of Leeds, UKWang, Shinn-Fwu, Ching Yun University, TaiwanWang, Wei-Chih, University of Washington, USAWang, Wensheng, University of Pennsylvania, USAWatson, Steven, Center for NanoSpace Technologies Inc., USAWeiping, Yan, Dalian University of Technology, ChinaWells, Stephen, Southern Company Services, USAWolkenberg, Andrzej, Institute of Electron Technology, PolandWoods, R. Clive, Louisiana State University, USAWu, DerHo, National Pingtung University of Science and Technology,TaiwanWu, Zhaoyang, Hunan University, ChinaXiu Tao, Ge, Chuzhou University, ChinaXu, Tao, University of California, Irvine, USAYang, Dongfang, National Research Council, CanadaYang, Wuqiang, The University of Manchester, UKYmeti, Aurel, University of Twente, NetherlandYu, Haihu, Wuhan University of Technology, ChinaYufera Garcia, Alberto, Seville University, SpainZagnoni, Michele, University of Southampton, UKZeni, Luigi, Second University of Naples, ItalyZhong, Haoxiang, Henan Normal University, ChinaZhang, Minglong, Shanghai University, ChinaZhang, Qintao, University of California at Berkeley, USAZhang, Weiping, Shanghai Jiao Tong University, ChinaZhang, Wenming, Shanghai Jiao Tong University, ChinaZhou, Zhi-Gang, Tsinghua University, ChinaZorzano, Luis, Universidad de La Rioja, SpainZourob, Mohammed, University of Cambridge, UKSensors & Transducers Journal (ISSN 1726-5479) is a peer review international journal published monthly online by International Frequency Sensor Association (IFSA).Available in electronic and CD-ROM. Copyright 2007 by International Frequency Sensor Association. All rights reserved.

Sensors & Transducers JournalContentsVolume 85Issue 11November 2007www.sensorsportal.comISSN 1726-5479Research ArticlesOptical Characterization of the Interaction of Mercury with Nanoparticulate GoldSuspended in SolutionKevin Scallan, Donald Lucas, and Catherine Koshland.1687Electrical Characterization of a Nanoporous Silicon Sensor for Low ppm Gas MoistureSensingTarikul Islam, Hiranmay Saha .1699Focused Ion Beam Nanopatterning for Carbon Nanotube Ropes based SensorVera La Ferrara, Ivana Nasti, Brigida Alfano, Ettore Massera and Girolamo Di Francia .1708Trace Moisture Response Property of Thin Film Nano Porous γ-Al2O3 for IndustrialApplicationDebdulal Saha, Kamalendu Sengupta .1714Gas Detectors Based on Single Wall Carbon Nanotubes by Exploiting the DielectrophoresisMethodLun-Wei Chang and Juh-Tzeng Lue .1721Detection of Hydrogen Sulphide Gas Sensor Based Nanostructured Ba2CrMoO6 Thick FilmsA. V. Kadu, N. N. Gedam and G. N. Chaudhari .1728Nanocomposites Sn-Si-O and Sn-Mn-O for Gas SensorsEkaterina Rembeza, Stanislav Rembeza.1739Theory and Instrumentation Related to Anomalous Dielectric Dispersion in OrderedMolecular GroupsTanmoy Maity, D. Ghosh and C. R. Mahata .1745Flexible Membrane Impact Sensor viaThick Film MethodHee C. Lim, James Zunino III and John F. Federici .1757Humidity Sensing Behaviour of Niobium Oxide: Primitive StudyB. C. Yadav, Richa Srivastava, M. Singh, R. Kumar and C. D. Dwivedi.1765Authors are encouraged to submit article in MS Word (doc) and Acrobat (pdf) formats by e-mail: [email protected] visit journal’s webpage with preparation instructions: .htmInternational Frequency Sensor Association (IFSA).

Sensors & Transducers Journal, Vol.85, Issue 11, November 2007, pp. 1687-1698Sensors & TransducersISSN 1726-5479 2007 by IFSAhttp://www.sensorsportal.comOptical Characterization of the Interaction of Mercurywith Nanoparticulate Gold Suspended in Solution1Kevin SCALLAN, 2Donald LUCAS, and 3Catherine KOSHLAND12Department of Mechanical Engineering, University of California at Berkeley,Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory,3School of Public Health, University of California at Berkeley, Berkeley, CA 94720Tel: 1-510-486 4134E-mail: [email protected]: 27 February 2007 /Accepted: 20 November 2007 /Published: 26 November 2007Abstract: We have demonstrated that the surface plasmon resonance (SPR) wavelength of goldnanoparticles suspended in solution can be modified by exposure to elemental mercury at sub parts permillion (ppm) concentrations in nitrogen. Ultraviolet-visible (UV-vis) absorption spectroscopy wasused to monitor the wavelength and maximum absorbance of the colloidal solution during and after theexposure process. Transmission electron microscopy (TEM) images revealed modifications to themorphology of the particles (size, shape, and extent of aggregation). The results show that the SPRwavelength is blue-shifted and the absorbance is increased with exposure time. After the exposure, thespectra were observed to relax toward their original position suggesting that the detection medium isregenerative. Copyright 2007 IFSA.Keywords: Mercury, Gold, Nanoparticle, Surface plasmon resonance1. IntroductionThe detection and quantification of mercury are important in many applications includingenvironmental monitoring, waste management, developmental biology, and clinical toxicology.Because of its persistent, bioaccumulative, and toxic (PBT) nature, mercury contamination hasemerged as a global concern. Elemental mercury, given its high vapor pressure (1.6 ppm on a massbasis at standard temperature and pressure, [1a]), high diffusivity in air (0.135 0.003 cm2/s, [2]),low water solubility (50 ppb on a mass basis at STP, [3]), and relative stability is difficult to capture,can be transported long distances, and has an atmospheric residence time of between 1-2 years.1687

Sensors & Transducers Journal, Vol.85, Issue 11, November 2007, pp. 1687-1698Gaseous (elemental) mercury is the dominant atmospheric species [4, 5]. It is removed from theatmosphere by dry deposition onto surfaces or by wet deposition after oxidation to water-solublemonovalent or divalent mercury. Divalent mercury is more stable and thus more common in theatmosphere, and is typically the primary component in oxic, suboxic, and anoxic aqueousenvironments [6]. It can be associated with inorganic molecules including chlorine, sulfur, andhydroxyl ions, and organic molecules giving rise to monomethylmercury and dimethylmercury, bothof which are highly toxic and can bioaccumulate by up to a factor of 105 in the aquatic food chain [7].Monomethylmercury is the most significant species in terms of adverse biological effects but typicallyonly represents a small fraction of the total mercury in any medium. However, given that all species ofmercury can be converted to the monomethyl compound, a quantitative understanding of theenvironmental fate of the species is critical to controlling, regulating, and assessing the ecological riskof mercury contamination. In particular, the control and regulation of gaseous or elemental mercury isof fundamental importance. The primary source of elemental mercury is anthropogenic emissions fromcoal-fired power plants and waste incineration facilities [8]. Recently, new regulation aimed atminimizing the human health effects and environmental hazards caused by mercury pollution hasnecessitated the need for sensitive, reliable, portable, and inexpensive mercury detectors [9].Commercial mercury analyzers are primarily based on atomic absorption spectroscopy, atomicfluorescence spectroscopy, or inductively coupled plasma mass spectrometry. These techniques, whilesensitive, generally lack portability, are expensive, and often require analyte preconcentration onto thesurface of a noble metal. In principle, colorimetric methods have the potential to satisfy therequirements for a simple, real-time, portable continuous mercury emissions analyzer but given thatmost conventional molecular dyes exhibit relatively low extinction coefficients, the challenge is toidentify appropriate binding leuco dyes capable of yielding sufficiently intense absorbing metal/dyecomplexes [10]. An alternative is to use nonmolecular chromophores such as free-electron metalnanoparticles that display visible extinction coefficients up to several orders of magnitude higher, e.g.:gold, silver, and copper.In this paper, we demonstrate the potential for a sensitive, reliable, portable, inexpensive elementalmercury detector that takes advantage of the visible surface plasmon resonance (SPR) wavelength ofgold nanoparticles. We first hypothesized that the SPR phenomenon can be used as an analytical toolto detect and quantify the concentration of elemental mercury adsorbed and/or absorbed to goldnanoparticles suspended in solution. Aqueous suspensions of the gold particles display an intenseplasmon absorption band centered at approximately 520 nm that renders the colloidal solution ruby-redin color. Gold nanoparticles were chosen for a number of reasons. First, as is well known in the miningindustry, mercury has a high affinity for gold and will readily form an amalgam [11]. Second, goldnanoparticles display a surface plasmon resonance (SPR) band in the visible region at about 520 nm.The exact location of the SPR wavelength is a complex function of particle morphology, thesurrounding medium, and any adsorbent species present [12, 13]. By maintaining a constant particlesize and shape, and stable surrounding medium, the degree to which a particular adsorbent, forexample elemental mercury, is adsorbed by the particles can be directly related to the change in theSPR wavelength. A further advantage of using gold nanoparticles is their very high surface area tovolume ratio which minimizes the amount of material required and maximizes the sensitivity.2. Material and MethodsColloidal gold was procured from British Biocell International (BBI), product code EM.GC5. Theparticle diameter was 5 nm with coefficient of variation 15% (0.75 nm) and particle roundnessgreater than 95%. The particle concentration was calculated to be 5 x 1013 particles per ml with amolar absorptivity of 2058 M-1 cm-1 at the SPR peak. The solution was stored in a transparent,polyethylene terephthalate (PETE) container at room temperature and remained stable for the duration1688

Sensors & Transducers Journal, Vol.85, Issue 11, November 2007, pp. 1687-1698of the experiment. Elemental mercury (99.9999% electronic grade from Aldrich Chem. Co.) was usedas the source of mercury vapor.The experimental apparatus is shown in Fig. 1. A bead (1 gram) of elemental mercury was indirectlysubmerged into a temperature controlled water bath and its vapor pressure controlled by setting thetemperature of the bath. The mercury vapor was entrained in a nitrogen (99.999% from Airgas) carrierstream flowing at 140 cubic centimeters per minute (ccm) and was assumed to be in a state ofequilibrium. The mercury vapor was exposed to the gold nanoparticles by bubbling the carrier streamthrough a series of three 4 ml UV-vis cuvettes with a 1 cm path length. Each cuvette contained 3.2 mlof the colloidal gold solution.Fig. 1. The experimental apparatus consisting of three cuvettes, a water bath to control the temperature, and abead of elemental mercury. The carrier stream was permitted to escape from the system after the last cuvette(i.e. #3, the left most cuvette).The experimental apparatus enabled us to relate the sensitivity of changes in the SPR wavelength tothe amount of mercury bubbled through the system, and to quantify the capture efficiency (defined asthe ratio of the number of mercury atoms captured by the colloidal solution to the total number ofatoms bubbled through the solution).3. CharacterizationA Perkin Elmer Lambda 2 UV-vis spectrometer was used to record the absorption spectra of thecolloidal solutions during and after the exposure. Standard 1 x 1 x 4 cm3 high density polyethylene(HDPE) UV-vis cuvettes were used and the background was set to pure (18.2 MΩ.cm) ambienttemperature distilled water from a Millipore Milli-Q ultrapure water purification system connected inseries with a Barnstead Fi-Stream type II distillation glass still. A FEI model Technai 12 TEM wasused to image the particles. The TEM samples were prepared on carbon coated 200 square-mesh TEMgrids (from SPI Supplies) by allowing a drop of the colloidal solution to dry by evaporation from thegrid surface.4. Results and DiscussionThe samples were characterized by UV-vis absorption spectroscopy and TEM imaging. Fig. 2(a)shows the UV-vis spectrum of the colloidal solution, as received. A TEM image of the particles isshown in Fig. 2(b). The broad absorption band centered at 519 nm (2.4 eV) is a result of the collectiveoscillation of free conduction electrons in response to an electromagnetic field [13-16]. The positionand width of the band depends on several factors including single particle properties (e.g. surfacefunctionality, adsorbate effects, electron density, etc.), the refractive index and viscosity of thesurrounding medium, particle concentration or average distance between neighboring particles, and1689

Sensors & Transducers Journal, Vol.85, Issue 11, November 2007, pp. 1687-1698temperature [12,13]. Provided these variables are known, the position of the resonance band maximumcan be used to assess colloidal concentration and particle size in solution [17].Absorbance (A)0.9SPR peak at(519 nm, 0.66 A)0.60.30.0400550700Wavelength [nm](a)(b)Fig. 2. (a) UV-vis absorption spectrum of the colloidal gold solution illustrating the SPR peak at 519 nm. Theabsorbance at the SPR peak is 0.66 A. (b) TEM image of the colloidal particles – the scale bar reads 100 nm.4.1. The Surface Plasmon Resonance PhenomenonThe SPR effect was first described quantitatively by classical electrodynamic (Mie) theory, based onbulk optical properties, by solving Maxwell’s equations with appropriate boundary conditions forsmall ( 100 nm) spherical particles [18]. For nanoparticles that are small compared to the wavelengthof the exciting electromagnetic radiation (2R λ, 2R 25 nm for gold [15]) the quasi-static ordiscrete dipole approximation can be used and the extinction (absorption plus scattering) cross sectionis given by [13, 14, 19]:σ (ω ) 9Vo ε m 3 2ε ′′,c [ε ′ 2ε m ]2 ε ′′ 2ω(1)where Vo is the spherical particle volume, c is the speed of light, ω is the angular frequency of theexciting radiation, εm the dielectric constant of the surrounding medium (assumed to be frequencyindependent) and ε′ and ε″ are the real and imaginary parts of the complex dielectric function of theparticle material respectively. From (1), Mie theory predicts that the surface plasmon resonance bandoccurs when ε′ -2εm (i.e., at the Fröhlich frequency) provided ε″ is small and only weakly dependenton the frequency. Further, (1) predicts that the position and width of the plasmon band are determinedsolely by ε″ and are independent of size, except for a varying intensity due to the volume term.In practice, however, a clear size-dependence is observed [13, 14, 20-23]. To account for thesefindings, basic Mie theory has evolved to include the fundamental assumption that the dielectricfunction of the nanoparticle material is size dependent (i.e. the intrinsic size-effect). The need tointroduce this size-dependence has been proposed to occur for free-electron metal particles when theirsize becomes smaller than the electronic mean free path in the bulk metal, approximately 20 nm forgold [13]. Using (1) and the bulk optical properties of gold [1b], the UV-vis spectrum of fivedifferently sized colloidal gold particles suspended in water was calculated. The spectra, normalized to1690

Sensors & Transducers Journal, Vol.85, Issue 11, November 2007, pp. 1687-1698one at the SPR wavelength, are shown in Fig. 3(a). For reasons not fully justified (see [22], p. 162),only the imaginary component of the particle dielectric function was adjusted for size (following themethod of Hovel et al. [24]). The result is a 1/R dependence of the plasmon bandwidth on particle size,in agreement with experimental results, and a fixed SPR wavelength.1.21.20.90.9ExtinctionAbsorbance (A)For larger particles, the extinction cross section is also size dependent and is accurately described bythe full Mie equation. The size-dependence is a result of changes to the real part of the dielectriccomponent of the particle material. In general, ε′ is an increasing function of frequency; when theparticle size is increased the absorption maximum is red-shifted. This is known as the extrinsic sizeeffect and is used extensively in the sizing of metal particles by optical extinction spectroscopy [22].Shown in Fig. 3(b) are the normalized extinction cross sections of four differently sized goldnanoparticles suspended in water. The spectra were calculated using the full Mie equation, thenumerical code in Bohren & Huffman ([16], Appendix A), and bulk optical properties. Clearly, theplasmon bandwidth incr

Sensors & Transducers Volume 85 Issue 11 November 2007 www.sensorsportal.com ISSN 1726-5479 Editor-in-Chief: professor Sergey Y. Yurish, phone: 34 696067716, fax: 34 93 4011989, e-mail: [email protected] Editors for Western Europe