In situ detection and removal of metal ion by porous gold electrode Cheon Seok Oh a , Hero Kim a , Selvaraj Rengaraj b , Younghun Kim a,⇑ a Department of Chemical Engineering, Kwangwoon University, Seoul 139-701, Republic of Korea b Department of Chemistry, Sultan Qaboos University, Muscat 123, Oman article info Article history: Received 23 May 2011 Received in revised form 29 June 2011 Accepted 12 August 2011 Available online 22 August 2011 Keywords: Porous gold Template Mercury ion Sensor Metal ion detection abstract A sensing electrode for the detection of heavy metal ions in aqueous solution selectively measured the concentrations of target materials on its functionalized surface, which has affinity to target metal ions. Target ions were adsorbed simultaneously on the functionalized electrode during the sensing process. Therefore, to understand this, experiments on the amperometric response and isotherms with an initial concentration of Hg 2+ were tested. Detection current was dependent on the concentration of Hg 2+ , and the equilibrium concentration of Hg 2+ adsorbed to the electrode showed a Langmuirian shape. Correla- tion between the detection current and removal capacity for Hg 2+ revealed that it is possible to estimate the adsorbed concentration on the electrode during the sensing step. Although the macroporous gold electrode prepared herein showed relatively low adsorption performance compared to conventional adsorbents, when we prepare nanoporous gold electrodes with a uniform nanopore structure and large surface area, in situ detection and simultaneously removal of metal ions by nanoporous gold electrode will be possible. Ó 2011 Elsevier Inc. All rights reserved. 1. Introduction Commonly used adsorbents for detection of heavy metal ions in aqueous solutions have micro- to macro-porous structures with larger surface areas, but conventional adsorbents such as charcoal and clay show inherent disadvantages; irregular pore structure, low selectivity, and low uptake capacity for metal ions [1]. There- fore, some promising ceramics with nanoporous structures (meso- porous silica and alumina) were developed via templating method [2,3], and their uptake capacities for heavy metal ions were found to be several times larger than those of conventional adsorbents. The uptake capacity of these adsorbents is calculated based on the difference between the initial and final concentration of target ions, which is measured by ICP spectroscopy (post-analysis) or sensing electrode (in situ analysis). Colorimetric sensor (e.g. preg- nancy tester) or electrode-system (e.g. pAg meter) has been sug- gested for real-time and in situ analysis of target materials. However, a colorimetric sensor can be used only once for qualita- tive (i.e. presence of target ions and range of concentration) not quantitative investigation [4,5]. Therefore, an electrochemical elec- trode system is generally used in field applications. The adsorption procedure is generally separated from the sens- ing procedure. To detect and simultaneously remove target materi- als in liquid phase, it is helpful that the detecting electrodes have porous structure (large surface area), functional groups (selective detection), and electrical conductivity (electrochemical detection). Therefore, herein, we proposed an in situ detective and adsorptive electrode composed of porous gold (PAu) based on previous reports [6,7]. Porous metal is generally fabricated by selective dealloying or templating method. In the dealloying method, white-gold (Au–Ag) alloy is employed, and Ag elements are selectively removed by chemical etching [8,9]. In the templating method, multi-processing steps are involved. The metal is deposited into the artificial tem- plates with different sizes, followed by removal of the templates to form the porous structure with an adjustable pore size [10]. In our previous report, porous gold electrode was shown to be very effective and promising for the development of high performance electrochemical and biological sensors. PAu/ITO electrode function- alized with thiol groups (HDT, 1,6-hexanedithiol) have been suc- cessfully applied to the detection of mercury ion at very low concentration and display high linearity from 7 to 150 ppb [6]. In addition, Pt nanoparticle-deposited PAu-pellet electrode also exhibits an extremely low working potential, a detection limit of 50 lM of H 2 O 2 , and a fast response time within 10–20 s [11]. As a proof-of-concept test, the submicron-sized porous gold electrode prepared by the templating method was used to detect and simultaneously remove mercury ion from aqueous solutions. For electrochemical sensing, PAu/ITO substrate was modified with functional groups and then evaluated by measuring the current according to concentration of mercury ion. When we used the cor- relation curve for uptake capacity (mg/g) by HDT/PAu/ITO elec- trode with detection current, equilibrium uptake capacity was estimated. The results of these experiments will be helpful to 1387-1811/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2011.08.019 ⇑ Corresponding author. Tel.: +82 2 940 5768; fax: +82 2 941 5769. E-mail address: korea1@kw.ac.kr (Y. Kim). Microporous and Mesoporous Materials 147 (2012) 1–4 Contents lists available at SciVerse ScienceDirect Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso