Sensors that are capable of reliable, selective and sensitive detection of organic species and functional groups are highly desirable for incorporation into conventional spectrophotometers. Fluorescence has been adopted as the spectroscopic technique of choice for optical sensor development due to its sensitivity and the opportunity for simultaneous multidimensional analysis using parameters of wavelength, intensity, polarization and lifetime. In combination these analytical parameters can be used to define unique solutions to both the qualitative and quantitative aspects of analysis. These techniques are useful for the direct and continuous detection of organic species in process streams-such as effluent streams.
Two problems associated with fluorescence spectroscopy as a strategy for the transduction of selective binding interactions is the relatively small absolute intensity that is available and the inability to use smooth metal substrates. Efforts have been made to overcome the weak signal using ultrasensitive detection equipment, powerful pulsed lasers, and by signal integration over extended periods of time. Such solutions however are costly and have tended to limit the potential for development of sensitive, small, low cost chemical sensors based on fluorescence spectroscopy. The fluorescence quenching effects of metal films has prevented the use of metals in conjunction with fluorophores for the facile self-assembly of rugged ultrathin chemically selective organic layers[1].
Metal island films composed of randomly distributed particles whose dimensions are small compared to the wavelength of visible light exhibit absorption bands in the visible region of the electromagnetic spectrum [2] as a result of the collective oscillation of conduction band electrons(surface plasmons). Metal island films can be used to enhance the analytical signal available from chemically selective membranes utilizing a fluorescence transduction scheme and can form the basis for the self-assembly of ultrathin organic films at surface.
Basis of interaction:
A molecular fluorophore can be modeled [3] as a four-state system consisting of two electronic states S.sub.o and S.sub.1 which each contain two vibrational levels, a zero-point and an excited state (*). The relative energy levels of these states are S.sub.o &lt;S.sub.o *&lt;S.sub.1 S.sub.1 *. Molecular fluorescence involves the absorption of a photon and the promotion of an electron from the ground state S.sub.o to the excited state S.sub.1 *, followed by thermal relaxation(transfer) to state S.sub.1 and then radiative emission to the vibrationally excited ground state S.sub.o *. The fluorescence yield (F) of the system is a function of the absorption rate (.OMEGA.), the transfer yield (Y.sub.trans) and emission yield (Y.sub.em) as shown in equation 1. EQU F=.OMEGA.Y.sub.trans Y.sub.em ( 1)
The absorption rate is indicative of the rate at which the excited state is populated and is a function of the cross-section of capture (.sigma.) of the fluorophore times the square of the local electric field strength (E.sub.local.sup.2). EQU .OMEGA.=.sigma.E.sub.local.sup.2 ( 2)
The effective population of the vibrationally relaxed excited electronic state S.sub.1 is characterized by the transfer yield, which is the ratio of the thermalization rate (T.sub.S.sbsb.1.spsb.*) (from state S.sub.1 * to state S.sub.1) to the total relaxation rate of S.sub.1 *. ##EQU1## The total relaxation rate from state S.sub.1 * is the sum of the thermalization rate (T.sub.S.sbsb.1 *) and the electronic radiative (R) and nonradiative (NR) relaxation rates (.GAMMA..sub.S.sbsb.1.spsb.* =.GAMMA..sub.S.sbsb.1.spsb.*.sup.R +.GAMMA..sub.S.sbsb.1.spsb.*.sup.NR).
The emission yield of state S.sub.1 is the ratio of the radiative decay rate to the sum of all the radiative and non-radiative decay rates from state S.sub.1. ##EQU2## This value is indicative of the quantum yield of the emitting state and is always smaller than or equal to 1.
An evanescent electric field is produced at the surface of a small metal particle such as silver, gold, indium, or alloys of mixtures of said metals which is significantly enhanced over that of the incident field, as a result of the coherent motion of electrons associated with the surface plasmon. The electric field enhancement surrounding a nonspherical particle is not uniform and depends on the shape of the particle. This enhancement, however, will be most intense in regions of high curvature. A phenomenological relationship developed by Weitz et al. [4] relates the fraction of light absorbed, A(.lambda.), at wavelength .lambda. (in vacuum), by the island film to the average field enhancement (f) at the particle surface as ##EQU3## where q is the volume fraction of the film, t the film thickness .epsilon..sub.1 +i.epsilon..sub.2 is the bulk complex dielectric constant of silver metal particles in the visible electromagnetic spectrum and .epsilon..sub.1 &lt;0,.epsilon..sub.2 &gt; and .vertline..epsilon..sub.1 .vertline.&lt;.epsilon..sub.2. The local electric field intensity decreases with distance (d) from a spherical particle [5] of radius (r) as ##EQU4##
When a fluorophore is located within approximately two particle diameters of a metal island [6] and its absorption and fluorescence bands overlap the absorption band of the metal island film, the fluorophore will then experience a large electric field amplification at the excitation and emission wavelengths. If f.sub..lambda.ex and f.sub..lambda.em are the enhancement factors associated with the electric field at the excitation and emission wavelengths respectively then the absorption rate is given by: EQU .OMEGA.=.sigma..sub..lambda.ex.sup.2 f.sub..lambda.ex.sup.2 E.sub.inc.sup.2( 7)
and similarly the radiative emission rate is enhanced over the spontaneous emission rate (.GAMMA..sub.spont.sup.R) as EQU .GAMMA..sup.R =f.sub..lambda.em.sup.2 .GAMMA..sub.spont.sup.R( 8)
Close proximity of the fluorophore to the metal surface provides an alternative non-radiative relaxation mechanism resulting in the production of electron-hole pairs or surface plasmons, which can increase the non-radiative decay rate of the fluorophore. This non-radiative energy transfer process involves dipole-dipole interactions which are described by a Forester-energy transfer process. As a result, the non-radiative relaxation rate depends on the distance of the fluorophore from the surface of the metal, and on the overlap of the fluorescence emission profile of the fluorophore and the absorption profiles of the island metal film. The non-radiative decay rate has been predicted to decreases with distance (d) from the surface [5] approximately as ##EQU5## The subsequently (electronically) excited state of the metal particle can relax radiatively or non-radiatively. For small particles (r&lt;150 .ANG.) [6] resistive heating (Joule heating) as a result of fluctuating electric fields within the metal island provides a significant non-radiative decay channel. Hence by controlling the particle size the type of signal which is to be measured (heat or fluorescence emission) can be optimized.