MIP films

An early example of an MIP-QCM sensor was a glucose monitoring system by Malitesta et al. (1999). A glucose imprinted poly(o-phenylenediamine) polymer was electrosynthesized on the sensor surface. This QCM sensor showed selectivity for glucose over other compounds such as ascorbic acid, paracetamol, cysteine, and fructose at physiologically relevant millimolar concentrations. A unique QCM sensor for detection of yeast was reported by Dickert and coworkers (Dickert et al. 2001 Dickert and Hayden 2002). Yeast cells were imprinted in a sol-gel matrix on the surface of the transducer. The MIP-coated sensor was able to measure yeast cell concentrations in situ and in complex media. A QCM sensor coated with a thin permeable MIP film was developed for the determination of L-menthol in the liquid phase (Percival et al. 2001). The MIP-QCM sensor displayed good selectivity and good sensitivity with a detection limit of 200 ppb (Fig. 15.7). The sensor also displayed excellent enantioselectivity and was able to easily differentiate the l- and D-enantiomers of menthol. 

Molecularly imprinted polymer (MIP) films can be utilized in conjunction with CdSe PL changes to enhance the selectivity of the sensor. MIPs have been developed to mimic the selective binding of molecules to biological substrates, as in... 

The use of MIP films with emissive semiconductor substrates can potentially impart selectivity to PL and EL responses. To illustrate, in addition to the PL enhancements caused by ammonia and trimethylamine (Figs. 2a and 2b), other amines like methylamine and dimethylamine elicit a similar PL response when adsorbed onto the bare surface of etched n-CdSe. These amines cannot generally be reliably distinguished with this technique because of the similarity of their PL... 

Poly(acrylic acid) (PAA), a MIP film candidate, has been shown to bind to the bare CdSe surface from methanol solution with considerable affinity [13]. Placement of drops of a PAA-methanol solution on the surface of CdSe and evaporation of the solvent leaves a PAA film on the semiconductor surface. Once coated with this PAA film, CdSe shows no change in PL intensity in the presence of amines. Despite the lack of a PL change, the deprotonation of the carboxylic acid could be observed by the shifting of the infrared (IR) carboxylic acid peak to lower frequencies characteristic of the carboxylate anion upon amine binding, as shown in Fig. 5. The reaction chemistry is ... 

Fig. 12 Illustration outlining MIP film fabrication. The C-terminus nonapeptide epitope is attached through a tether to a glass or oxidized silicon surface by the N-terminal amino acid of the peptide. Monomers are photochemically cross-linked while remaining in contact with the peptide modified surface. Following polymerization, the glass substrate is removed. The protein can now bind to the MIP via its C-terminus nonapeptide epitope. Modified from [114]...

MIP film included between two Arrangement of MIP particles between membrane layers two membrane layers... 

A few studies have reported the embedding of an MIP film between two membranes as a strategy for the construction of composite membranes. For example, a metal ion-selective membrane composed of a Zn(II)-imprinted film between two layers of a porous support material was reported [253]. The imprinted membrane was prepared by surface water-in-oil emulsion polymerisation of divinylbenzene as polymer matrix with 1,12-dodecanediol-0,0 -diphenylphosphonic acid as functional host molecule for Zn(II) binding in the presence of acrylonitrile-butadiene rubber as reinforcing material and L-glutamic acid dioleylester ribitol as emulsion stabiliser. By using the acrylonitrile-butadiene rubber in the polymer matrix and the porous support PTFE, an improvement of the flexibility and the mechanical strength has been obtained for this membrane. 

Scheme 2 Flow chart of the procedure for preparation of an MIP film by surface grafting...

Sandwich casting permits one to prepare an MIP film with uniform thickness [28, 106, 108, 109]. In this procedure, a drop of the solution containing a monomer, cross-linker, template and initiator is dispensed on the surface of a PZ transducer and covered with a microscope quartz slide. Then this assembly is exposed to UV light in order to initiate polymerization that results in a thin MIP film. The polymerization can be performed either on the activated immobilized initiator PZ transducer surface or on the bare transducer surface. For example, sialic acid has been determined with an MIP film immobilized on a platinum-film electrode of the quartz resonator using the former procedure [57]. That is, 1-butanethiol has been used for modification of the Pt surface. An indole-3-acetic acid plant hormone served as the template. An MIP-PZ chemosensor prepared that way operated reproducibly. That is, the coefficient of variation of the chemosensor performance was 9% for three different sensors. 

In order to minimize swelling, MIP is often synthesized in a polar solvent, such as dimethylformamide (DMF) or ACN [113]. Most importantly, a (cross-linker)-to-(functional monomer) mole ratio affects the chemosensor performance. That is, a high (5 1) ratio enhances mechanical strength of an MIP film and favours formation of binding sites. But a low (1 5) ratio leads to a flexible MIP film of improved template memory and stronger adherence to the transducer surface [114]. Therefore, this ratio must be compromised for preparation of MIP of superior performance [114]. 

For instance, aromatic solvent vapours were determined with polyurethane MIPs combined with SAW transducers [124]. That is, first, the hydrophilic quartz surface of SAW was hydrophobized with NW-dimethylaminotrimethylsilane. Then a solution for polymerization was prepared by mixing functional monomers, such as 4,4 -dihydroxydiphenyldimethylmethane, 4,4 -diisocyanatodiphenylmethane and 30% 2,4,4 -triisocyanatodiphenylmethane, with the 1,3,5-trihydroxybenzene crosslinker in the ethyl acetate or ethanol template used also as the solvent for polymerization. Subsequently, the hydrophobized resonator surface was spin-coated with an aliquot of this solution. Finally, the free-radical polymerization has been initiated thermally to form a polyurethane MIP film. The desired vapour concentration and relative humidity of the analyte were achieved by mixing dry air and saturated steam with solvent vapours generated with thermoregulated bubblers. 

The chemosensor response to the solvent, which was used for imprinting, was higher than that to other solvents. For instance, the MIP film, prepared using ethyl acetate as both the polymerization solvent and template, more strongly bound ethyl acetate than the ethanol interferant. The amount of the cross-linker played a decisive role in discrimination of this interferant. The sensitivity and selectivity of the 433-MHz MIP-SAW chemosensor and 10-MHz MIP-QCM chemosensor were compared. It appeared that affinity for 1,2-xylene of the SAW chemosensor using the 1,2-xylene imprinted MIP film was pronounced. That is,... 

Most SAW transducers are inadequate for operation in liquids because most types of surface acoustic waves are completely damped in this viscous environment. Therefore, QCM transducers are used instead. For successful operation of an MIP-QCM chemosensor in liquid, the MIP film should be sufficiently stable with respect to dissolution and peeling off from the resonator surface. Moreover, this film should be relatively rigid, neither shrinking nor swelling in the test solution. 

An MIP film attached to a quartz resonator of the QCM transducer has been used successfully to devise a chemosensor for determination of disinfection by-products of haloacetic acids in drinking water [128]. An ACN solution of trichloroacetic acid (TCAA), VPD, EGDMA and AIBN, used as the template, functional monomer, cross-linker and initiator, respectively, has been polymerized thermo-radically. Next, the resulting MIP was spin-coated on a quartz resonator to form a thin film. The TCAA template was then extracted by rinsing the film with water. This TCAA... 

Biogenic amines, such as histamine [131], adenine [132], dopamine [133] and melamine [134], have been determined using chemosensors combining MIP recognition and PM transduction at QCM. Electronically conducting MIPs have been used in these chemosensors as recognition materials. Initially, functional electroactive bis(bithiophene)methane monomers, substituted either with the benzo-18-crown-6 or 3,4-dihydroxyphenyl, or dioxaborinane moiety, were allowed to form complexes, in ACN solutions, with these amines as templates. Subsequently, these complexes were oxidatively electropolymerized under potentiodynamic conditions. The resulting MIP films deposited onto electrodes of quartz resonators were washed with aqueous base solutions to extract the templates. 

The histamine [131], adenine [132] and dopamine [133] amines are electroactive in the positive potential range, in which the thiophene is electropolymerized. Therefore, these amines could be oxidized at the electrode surface in the course of deposition of the MIP film. That way, products of these oxidations might be available in the electrode vicinity for imprinting rather than the desired pristine... 

Thickness of the barrier layer, optimized at 220 nm [133], played a crucial role with respect to the chemosensor sensitivity, selectivity and LOD. So, eventually, the chemosensor architecture comprised a gold-film electrode, sputtered onto a 10-MHz resonator, coated with the poly(bithiophene) barrier layer, which was then overlaid with the MIP film. This architecture enabled selective determination of the amine at the nanomole concentration level. LOD for histamine was 5 nM and the determined stability constant of the MIP-histamine complex, XMn> = 57.0 M 1 [131], compared well with the values obtained with other methods [53, 136, 137]. Moreover, due to the adopted architecture, the dopamine chemosensor could determine this amine with the stability constant for the MIP-dopamine complex, XMip = (44.6 4.0) x 106 M-1 and LOD of 5 nM [133], which is as low as that reached by electroanalytical techniques [138]. The MIP-QCM chemosensor for adenine [132] also featured low, namely 5 nM, LOD and the stability constant determined for the MIP-adenine complex, XMIP = (18 2.4) x 104 M, was as high as that of the MIP-adenine complex prepared by thermo-induced co-polymer-ization [139]. The linear concentration range for determination of these amines extended to at least 100 mM. 

Fig. 4 Resonant frequency changes with time due to repetitive FIA melamine injections, for the MIP-QCM chemosensor. Melamine concentration is indicated with number at each curve. Inset shows FIA calibration plots for (1) melamine and its interfering compounds, such as (2) ammeline, (3) cyanuric acid, and (4) cyromazine. Volume of the injected sample solution was 100 pL. The flow rate of the 1 mM FIC1 carrier solution was 35 pL min-1. The MIP film was prepared by electropolymerization of 0.3 mM bis(2,2 -bithienyl)-benzo-[18-crown-6]methane functional monomer and 0.3 mM 3,3 -bis[2,2 -bis(2,2 -bithiophene-5-yl)]thianaphthene cross-linking monomer, in the presence of 0.1 mM melamine, in the trihexyl(tetradecyl)phosphonium tris(pentafluor-oethy 1)-trifluorophosphate ionic liquid ACN (1 1 v/v) solution, which was 0.9 mM in trifluoroacetic acid (pH = 3.0). The melamine template was extracted from the MIP film with 0.01 M NaOH before the determinations (adapted from [134])...

A molecularly imprinted polypyrrole film coating a quartz resonator of a QCM transducer was used for determination of sodium dodecyl sulphate (SDS) [147], Preparation of this film involved galvanostatic polymerization of pyrrole, in the presence of SDS, on the platinum-film-sputtered electrode of a quartz resonator. Typically, a 1-mA current was passed for 1 min through the solution, which was 0.1 mM in pyrrole, 1 mM in SDS and 0.1 M in the TRIS buffer (pH = 9.0). A carbon rod and the Pt-film electrode was used as the cathode and anode, respectively. The SDS template was then removed by rinsing the MlP-film coated Pt electrode with water. The chemosensor response was measured in a differential flow mode, at a flow rate of 1.2 mL min-1, with the TRIS buffer (pH = 9.0) as the reference solution. This response was affected by electropolymerization parameters, such as solution pH, electropolymerization time and monomer concentration. Apparently, electropolymerization of pyrrole at pH = 9.0 resulted in an MIP film featuring high sensitivity of 283.78 Hz per log(conc.) and a very wide linear concentration range of 10 pM to 0.1 mM SDS. 

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