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Ind. Eng. Chem. Res. 2006, 45, 656-662
Bowing Effect with Fluorescence: A Unique Chemosensor for the Silver Ion Dong Hun Shin Electronic Components Group, R&D Center, LS Cable, Hogye-dong 555, Dongan-gu, Anyang-si, Kyungki-do, 431-080, Korea
Young Gun Ko and Ung Su Choi* Tribology Research Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, Korea
Woo Nyon Kim Department of Chemical and Biological Engineering, Korea UniVersity, Anamdong 5 ga-1, Seoul 136-701, Korea
A unique chemosensor for Ag ions was created by coupling an anthracene signaling unit onto an amineterminated glass slide. Chemical dye (Orange II) was applied to establish the exposed amine groups on the surface, because the coupling reaction is dependent on the amount of the exposed amine groups. The assembled layer had a surface that was not flat but, instead, had an embossed shape, on the microscale; the surface roughness was greater than that of the original glass, because of the morphology of the rare glass. The quenching effect of the synthesized chemosensor is notable in Ag aqueous solution, despite concentrations on the parts per billion (ppb) scale. When several Ag ions were provided for the chemosensor, the morphology of the chemosensor was considerably changed, which made the contact angles change. All the states of the chemosensor in the surface were characterized using atomic force microscopy (AFM) and scanning electron microscopy (SEM). The bowing effect and binding state, in regard to the adsorption of Ag ions, were further specified using X-ray photoelectron spectroscopy (XPS). 1. Introduction The application of a fluorescent chemosensor is a useful optical method to seek molecular recognition events. As a result, it has been applied to environmental and biological monitoring. A chemosensor can be defined as a molecule that contains an active unit, spacer, and a receptor unit.1 To achieve a strong chemical affinity between hosts and guests, which means selectivity, the design of the receptor unit is a great concern for the chemosensor. Especially in metal sensors, there are two main factors for selectivity: coordination number and radius of the metal ions.2 In view of the receptor units, the stabilization, in regard to the chelate effect, is highly dependent on the size, shape, and angle of the chelate ring (such as podand, corand, and cryptand).3 There are various chemosensors in terms of those types, such as polymer, powder, and surface-fabricated solid, which are used for special purposes.4-8 The most widely used active units contain anthracene and pyridine moieties, because of their strong fluorescent emission with high quantum yields.9,10 Nevertheless, limited empirical information is available about the factors that control the fluorescence properties of fluorescein. Nagano and his colleagues reported a rational design of the fluorescence structure, in regard to quantum yield.11 Recently, Raymo has carried chemical logic circuits or more using fluorescein.12 A self-assembled treated glass would be the best substrate for a chemosensor, because of its optical properties, pH, and thermal stability; therefore, glass slides were used as the chemosensor matrix in this paper.13,14 The chemical and physical properties of the self-assembled thin layer are of the utmost importance, because they crucially influence the mor* Author to whom correspondence should be addressed. Tel.: +822-958-5667. Fax: +82-2-958-5659. E-mail:
[email protected].
phology and surface density of the molecular assembled molecules. Naturally, they also have an effect on the reaction. To determine the absolute density of the self-assembled molecule, Park used a coupling reaction to imine formation between amine and aldehyde.15 In the same concept, Orange II solution was adopted to establish the surface density through pH control,16 because almost all of the amine groups are protonated at pH 3 and reversibly deprotonated at pH 11. After confirmation of the density, 9,10-bis(formamido)-anthracene (BFA) was coupled with an amine-terminated monolayer as an active unit and receptor. Several cations and concentrations were used to evaluate the metal binding properties, but only Ag(I) has a highly fluorescent quenching effect. The morphologic changes caused by self-assembly, coupling reactions, and silverrelated formation were confirmed using atomic force microscopy (AFM), scanning electron microscopy (SEM), and measurement of the contact angle. 2. Experimental Section General Information. Contact angles were measured using a face contact angle meter (model CA-D, Kyowa Interface Science Co.). Scanning electron microscopy (SEM) (model S-4200, Hitachi) and atomic force microscopy (AFM) (Park Scientific Instruments) were applied for surface characterization. Optical properties of the glass slides and Orange II solutions were identified by ultraviolet-visible light (UV-vis) spectrometry (model Operon-3000, Hanson Technology). To calibrate the UV-vis spectrometer, a clean glass slide was used as a reference. Further detailed binding states with Ag ions were examined via X-ray photoelectron spectroscopy (XPS) (model PHI 5800, Physical Electronics Instruments). Materials. 3-(Aminopropyl)triethoxysilane (APTES), toluene, ethanol, concentrated sulfuric acid, BFA, dimethyl sulfoxide
10.1021/ie051091h CCC: $33.50 © 2006 American Chemical Society Published on Web 11/23/2005
Ind. Eng. Chem. Res., Vol. 45, No. 2, 2006 657 Scheme 1. Salt Form of Orange II on an Aminosilanized Glass Slide
(DMSO), Orange II sodium salt, sodium borohydride, HCl (1 N), HNO3 (1 N), NaOH (1 N), KNO3, AgNO3, Pb(NO3)2, ZnCl2, CrCl2‚6H2O, Ni(NO3)2‚6H2O, and CuCl2‚6H2O were purchased from Aldrich Chemical Co. and used as received. Finely grounded glass slides were purchased from Marienfeld, and deionized (DI) water was used. Preparation of the Amine-Terminated Glass Substrate. Glass slides were treated using a piranha solution (1:3 30% H2O2/H2SO4) for 30 min at ambient temperature. Subsequently,
they were rinsed several times with DI water, sonicated in DI water once (10 min), rinsed several times with spectroscopicgrade ethanol, and sonicated in ethanol once (10 min). The remaining ethanol on the glass surface was removed using nitrogen gas. Dry toluene (300 mL) was added to the jar, which was equipped with a slot that contained the hydrolyzed glasses, followed by the addition of APTES (30 mL). The coupling reaction was allowed to proceed (the minimum reaction time allowed was ∼10 h) at 25 °C under a nitrogen gas atmosphere. No shaking or stirring occurred, to prevent any side effects. After the desired reaction time, the slides were taken out of the jar and rinsed three times with toluene, a mixed solution of toluene and ethanol (1:1), and ethanol, in that order. The assemblies then were dried in an oven at 120 °C for 1 h. The reaction was performed at least three times, to verify the reproducibility of the reaction through the XPS atomic ratio. Formation and Deformation of the Orange II Salt. Amineterminated substrates (3 cm × 2 cm) were immersed into a pHadjusted Orange II solution (pH 3, 20 mL) for 5 h at 30 °C and then washed with pH 3-adjusted HCl, to remove any unadsorbed dye, and blown with N2 gas. To desorb dye chemicals, the slide was immersed into pH 11-adjusted DI water (20 mL) for 5 h at 30 °C. The water that contained Orange II was used for back titration. Coupling Active Units onto the Amine-Terminated Glass. Modified glass slides were added into 0.1 mmol of BFA in
Figure 1. Ultraviolet-visible light (UV-vis) spectra of back-titrated results and the amount of exposed amine groups on the 1 nm × 1 nm square glass, according to the consecutive aminosilanization time.
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Ind. Eng. Chem. Res., Vol. 45, No. 2, 2006
Figure 2. Contact-mode atomic force microscopy (AFM) images of the molecularly assembled layer on slide glass: (a) bare glass, (b) treated for 1 h, (c) treated for 3 h, and (d) treated for 4 h. (The root-mean-square (rms) roughness values are 15.8, 25.1, 36, and 81 Å, respectively.) Merged small images on the 3 µm × 3 µm images have a magnitude of 0.5 µm × 0.5 µm.
Scheme 2. Simulated Molecular Structure and the Associated Contact-Angle Properties
DMSO (150 mL), and then the solution was stirred at 30 °C for 24 h with inert gas. The glass then was washed at least two times with ethanol and sonicated for 2 min. The glass was reacted once more with 1 mmol NaBH4 in ethanol (150 mL) at 30 °C to break the imine form on the glass surface. The glass was washed with DI water and sonicated for 2 min after 24 h. Quenching Effect. For the fluorescent emission tests, several metal ions (such as Na+, K+, Ag+, Pb2+, Zn2+, Cr2+, Ni2+, and Cu2+) were provided. Without pH adjustment, metal aqueous solutions (100 mL) with concentrations of 0.1 M, 0.01 M, 1 mM, 0.1 mM, 0.01 mM, 1 µM, 0.5 µM were tested, respectively. The dipping and stirring time of the chemosensor was
