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Microchim Acta (2009) 164:395–404DOI 10.1007/s00604-008-0073-7 Oxygen plasma-treated gold nanoparticle-based field-effectdevices as transducer structures for bio-chemical sensing Jenny Gun & Dan Rizkov & Ovadia Lev &Maryam H. Abouzar & Arshak Poghossian &Michael J. Schöning Received: 2 April 2008 / Accepted: 30 May 2008 / Published online: 4 July 2008 # Springer-Verlag 2008 Abstract EIS (electrolyte-insulator-semiconductor) sensors based on the functionalization of uncoated gold nano-particles supported on a Si/SiO2 structure are presented.
Since they were first introduced to the sensor arena by Oxygen plasma etching at moderate power (<200 W) Brust et al. functionalized gold and precious metal provides a convenient and efficient way to remove organic nanoparticles have been increasingly used to enhance capping agents from the gold nanoparticles without electrochemical and photometric sensing applications significant damage. Higher power intensities destroy the Their high surface area, easy functionalization, high electric linkage between the SiO2 and the gold nanoparticles, and conductivity, high stability and corrosion resistance, and some of the gold nanoparticles are removed from the their pronounced plasmon resonance band in the visible surface. The flat-band potential shift, i.e. the pH depen- range as well as sensitivity to aggregation are amongst their dence of the gold-coated EIS sensors is similar (33 mV/ most attractive features. However, despite the popularity of pH) to the uncoated EIS pH-sensor. Lead, penicillin and gold nanoparticles in electrochemical and photoelectro- glucose sensors were prepared by immobilization of β- chemical sensing there are only a few articles describing the cyclodextrin, penicillinase and glucose oxidase by various use of metal nanoparticle-based field-effect sensors. A notable exception is the work of Willner's laboratory to detect neurotransmitters and DNA by modified gold Keywords Field-effect device . Gold nanoparticles .
nanoparticle-coated alumina gate field-effect transistors Functionalization . EIS sensor . Penicillin . Glucose . Lead (FET). Willner's FET sensors utilize gold nanoparticlesembedded in a polymeric film (polyethyleneimine), whichhinders the accessibility of analytes to the functionalizedsurface of the nanoparticles. Presumably, the capping agentused to prevent aggregation of the nanoparticles in the J. Gun : D. Rizkov : O. Lev (*)Institute of Chemistry, The Hebrew University of Jerusalem, solution phase also limits the amount of the selective recognition elements that can be bound on the gold Jerusalem IL-91904, Israel nanoparticles. This drawback deters to some extent researchers from utilizing nanoparticles as a vector for incorporation of desirable functionalities in FET sensors.
Institute of Nano- and Biotechnologies (INB), In this article, we demonstrate a strategy to remove the Aachen University of Applied Sciences, Campus Jülich, protective capping agent moieties by oxygen plasma treatment. Oxygen plasma ashing involves RF (radio DE-52428 Jülich, Germany frequency) excitation of pure oxygen gas under an electric M. H. Abouzar A. Poghossian M. J. Schöning (*) field to give active oxygen species that effectively oxidize Research Centre Jülich, (ash) carbonaceous materials. The ash and residues are Institute of Bio- and Nanosystems (IBN2), removed by mechanical vacuum pumping. The process is DE-52425 Jülich, Germanye-mail: routinely used in electronic industries either under low J. Gun et al.
voltage to clean surfaces or under high power conditions penicillinase (EC, Bacillus cereus from Sigma) with (>100 W) to etch away undesirable polymeric films and 2,540 U/mg activity, penicillin, D-glucose, horseradish photoresists. Here, we adopt the same setup to completely peroxidase, HRP (type II), and o-dianisidine dihydrochloride remove the capping agents off the gold nanoparticles, and were purchased from Sigma-Aldrich.
to eliminate the thiol films that bind the gold nanoparticlesto the surface. As far as we know such a process was never Preparation of gold sol Self-stabilized gold sol in toluene reported before in the context of sensing applications. The was prepared by a slightly modified Brust procedure process was extensively used by Boyen and coworkers Fifty milliliters of 50 mM tetraoctylammonium bromide in for the removal of polymer coatings from gold nano- toluene was added to a 200 ml Erlenmeyer containing particles at 50 W oxygen plasma. Raiber et al. and 25 ml of a 40 mM aqueous solution of HAuCl4 and stirred Hesse and Creighton recently demonstrated that self- with a magnetic stirrer for 20 min. Then, the colorless assembled monolayers of alkanethiolates and other thiolates aqueous phase was separated in a separation funnel and can be effectively etched off gold surfaces by either discarded. The orange colored organic phase was returned hydrogen or oxygen plasma etching. The authors showed to an Erlenmeyer, and 25 ml of freshly prepared water that the treatment results in an Au2O3-covered surface. In solution containing 0.4 g of NaBH4 was added under our case, the residual gold oxide is removed by alcohol vigorous agitation. After the mixture was stirred for 2 h, the reduction, leaving an organic-free sensing element. Due to dark ruby organic phase was isolated and subsequently its compatibility with the electronic industry, for a future washed in aqueous 0.1 M H2SO4, then in 1 M Na2CO3, and practical application of nanoparticle-based field-effect finally in water. The organic phase was then dried by devices, this technique was preferred over other well- MgSO4 and filtered through a 0.45 μm PTFE syringe filter established wet chemistry processes (e.g., Pirahna, ozona- (MetaChem Technologies Inc., Torrance, CA; tion, electrochemical etching) for removal of gold thiolate The resulting sol contained approximately 12 mM gold with a mean particle diameter of 6–8 nm. The Four different key experiments have been performed in light absorption peak of the solution was at 523 nm order to exemplify the use of plasma-etched gold nano- wavelength (the optical density was 2.5–3.3 in a 2 mm3 cell) [The gold colloid solution was stored in 4 °C, no devices: pH sensing by bare gold nanoparticle-modified degradation was observed in half a year's time.
surfaces, lead ion-sensing by methylamine β-cyclodextrin-modified gold nanoparticles, penicillin and glucose sensingby adsorbed and covalently bonded enzymes (glucose Substrate preparation and gold deposition oxidase and penicillinase, respectively). Currently, the mainadvantage of using gold modified EIS stem from their Sensor preparation and cleaning Al–Si–SiO2 structures convenient use, but in a forthcoming publication we shall (300 nm aluminum film as a rear-side contact layer, p-Si, demonstrate that the same approach can be used to enhance ρ=5–10 Ωcm, 30 nm dry thermally grown SiO2) were cut glucose electrosensing into pieces of 10 × 10 mm2 and washed for 5 minsequentially in an ultrasonic bath containing the followingsolvents: acetone, propanol, water, 2% Hellmanex II Experimental section (Hellma, solution, and in distilledwater once more. The cleaned sensors were then dried with Reagents and materials N2, and treated with O2 plasma at 250 W for 10 s. (TePLa-100 E, Technics Plasma, Germany). The oxygen partial All chemicals were of reagent grade and were used as pressure was 1.42 mbar. The preliminary tests showed that a treatment with O2 plasma for a shorter time or with lower 95%, and hydrogen tetrachloroaurate were from Aldrich power resulted in an inhomogeneous surface, imperfect ), toluene and DMSO were subsequent silanization and eventually yielded poor gold from J.T. Baker ), and sodium borohydride was from Merck ().
Water was deionized (conductivity <0.1 μS) by a Seradest Sensor silanization and gold modification The sensor SD 2000 system (). For the silanization procedure was similar to the reported protocol pH measurements, technical buffer solutions (Titrisol, Dry sensor substrates were exposed to a freshly Merck) in the concentration range from pH 4 to pH 10 were prepared solution of MPTMOS in toluene (10 vol%) at used. Glucose oxidase (GOx; EC, type VII, from 20 °C for 1 h, rinsed five times with toluene, then three Aspergillus niger) with an activity value of 121 U/mg, times with isopropanol, and finally rinsed thoroughly with

Oxygen plasma-treated gold nanoparticle-based field-effect devices acetone. In order to stop further undesirable hydrolysis and thoroughly washed with distilled water and stored in condensation reactions in the newly formed films, the 4.5 mM PBS buffer, pH 7.5, when not in use.
sensors were washed for 10 min in 1 mM aqueous aceticacid solution and then rinsed with water and dried. A gold Surface characterization by ellipsometry, contact-angle nanoparticle monolayer was deposited on the silanized measurements, light-reflectance spectroscopy, and XPS samples by immersion of the sensor in toluene–gold sol for Thickness and homogeneity of the silane layer were 12 h. The sensors were then washed in toluene, isopropanol, characterized by imaging ellipsometry (IE) (EP3, Nanofilm acetone, and water.
Germany) equipped with a frequency doubled Nd:YAG laser(532 nm) and 60° incident angle. The imaging was Sensor treatment with oxygen plasma The sensor treatment conducted over an image area of 0.1 mm2 with a lateral with O2 plasma was done always for 10 s in microwave resolution of 2 μm. Ellipsometry was used to control the plasma. The oxygen partial pressure in nitrogen atmosphere thickness of the organosilane layer after every silanization was 1.42 mbar. The plasma power was a subject of the procedure. Optimal sensor performance was achieved when additional studies described below.
the thickness of the organosilane layer was 1.6–2.0 nm,which is equivalent to the thickness of two monolayers. The β-cyclodextrin (β-CD)-modified sensor Aminomethyl β- contact angle was measured with an OCA20 CA system cyclodextrin was synthesized according to ref. ]. The (Dataphysics, Germany) utilizing the Sessile drop method purity of the product was verified by H-NMR and with 5 μl Milli-Q distilled water. Each substrate was dried elemental analysis. The gold-modified surface was treated with nitrogen before the measurement. Light reflectance of with O2 plasma (10 s, 200 W) and then immersed in ethanol the gold-coated sensors was measured using a Cary 1E UV– overnight. Then, the sensor was left for 24 h in a water vis spectrophotometer (Varian) operated in the reflectance solution of aminomethyl β-cyclodextrin (50 mg/ml). The mode. SEM (scanning electron microscopy) images were sensor was ready for use immediately after careful water performed by Zeiss Gemini 1550 at 60°. XPS (X-ray wash to remove free aminomethyl β-CD.
photoelectron spectroscopy) measurements were done usingPHI 5600 equipped with a monochromatic AlKα radiation Modification of sensor with penicillinase Two methods were used to prepare the penicillin sensors: 1. Physical adsorption—3 mg of penicillinase were Electrochemical measurement setup dissolved in 1.5 ml of PBS buffer (pH 7.5, 20 mM);0.15 ml of this solution was pipetted onto the surface of The schematic cross-section of the EIS sensor and the the sensor and evaporated under nitrogen. The sensor measurement set-up is presented in Fig. The EIS sensor was washed with distilled water and kept in a was mounted into a home-made measuring cell, sealed by refrigerator when not in use.
an O-ring and contacted on its front side by the electrolyte 2. Chemical coupling—The gold nanoparticle-modified and on its rear side by a gold-plated pin. An Ag/AgCl sensor was washed twice with water, twice with DMSO electrode (3 M KCl, Metrohm) that was used as a reference and then left for 6 h in 4 mg/ml dithiobis(succinimy- electrode was mounted from the top side. The side walls dylpropionate) (DSP, from Pierce, Rockford, IL; ) in DMSO solution according tothe procedure of Pierce []. It was then washed twotimes with DMSO, two times with water and transferred into 2.1 mg/ml of penicillinase in PBS buffer (pH 7.5, 20 mM) for overnight storage at 4 °C. Next, the sensorswere thoroughly washed with distilled water and stored in 4.5 mM PBS buffer, pH 7.4 when not in use.
Modification of the sensor with glucose oxidase The gold nanoparticle-modified sensor was washed two times with water, two times with DMSO and left for 6 h in 4 mg/mlDSP in DMSO. It was then washed two times with DMSO,two times with water and transferred into 4.9 mg/ml ofGOx (Aldrich, 121 U/mg,) in NH4Ac 4.5 mM buffer, Fig. 1 Schematic cross-section of the EIS sensor modified with gold pH 6.8 for overnight storage at 4 °C. Next, the sensors were nanoparticles and measurement setup. RE reference electrode

J. Gun et al.
Fig. 2 Sensor preparation: step 1 bare Si/SiO2 sensor chip, step 2 silanization, step 3 gold colloid deposition, step 4 oxygen plasma treatment,step5 surface modification of gold nanoparticles and backside contacts of the EIS sensor chip were protected mounted into the measuring cell; before testing, the sensors from the electrolyte solution by means of an O-ring, were preconditioned in phosphate buffer solution for at least thereby circumventing the need for a complex encapsula- 2 h. Unless otherwise stated, the impedance measurements tion process. The contact area of the EIS sensor with the were carried out in 18 mM phosphate buffer, pH 7.4, in a solution is determined by the diameter of the O-ring and frequency range varying from 1 Hz up to 1 kHz. C–V was about 0.5 cm2. All measurements were performed in a measurements were carried out at 100 Hz frequency in the dark Faraday cage at room temperature. All potential values voltage range from −4 V to 1 V. The ConCap measurements are referred to the reference electrode.
were performed at a frequency of 100 Hz, unless otherwise The electrochemical characterization of the EIS sensors was performed by means of capacitance–voltage (C–V),impedance spectroscopy, and constant-capacitance (ConCap)methods using an impedance analyzer (Zahner Elektrik, Results and discussion Kronach). For operation, a direct current polarisation voltagewas applied via the reference electrode to set the working Sensor surface characterization point of the EIS sensor, and a small alternating currentvoltage (20 mV) was applied to the system in order to The fabrication of the sensor was a multistage process measure the capacitance of the sensor. The sensor chips were (Fig. involving silanization, gold colloid deposition, Fig. 3 Electrochemical charac- Initial sensor (1)
terization of the EIS sensor by means of C–V measurements Gold colloid modification (3)
after each preparation step. The O2 plasma treatment (4)
inset shows the shift of CD modification of gold surface (5)
flat-band voltage FBP shift, mV -400
Voltage, V

Oxygen plasma-treated gold nanoparticle-based field-effect devices Table 1 Contact angle and flat-band potential shifts (FBP) by different substrate treatments of the EIS structure Preparation steps Contact angle (°) plasma treatment, and specific surface modification of thebare gold nanoparticles (NP). In the following, thenecessity of each preparation step and the correspondingflat-band potential shifts of the EIS sensor associated withthe different modifications will be briefly discussed.
Changes in the capacitive characteristics (C–V curves) andflat-band potential (FBP) shifts can be seen from Fig. and are summarized in Table .
Notably, the silanization shifts the flat-band voltage by some 30 mV (Table ) in the negative direction due to apartial coverage of surface silanols with gold nanoparticles.
Carrying out the silanization procedure in a less dryenvironment or for a prolonged duration resulted in up toten times thicker films (as observed by the ellipsometricmeasurements) and positive FBP shift, underscoring theeffect of the silanol groups in the silicate coatings. As expected, the silanization step is also accompanied by asignificant surface hydrophobization, which is manifestedin an increase of the water wetting angle from 3° to 62°.
The subsequent addition of gold nanoparticles shifts thepotential back into positive direction. A possible explana-tion for this observed shift is the cumulative effect ofsurface silanols blocked by negatively charged NP. Thenegative charge of the gold NP was reported before, and itis attributed to the thiol–gold capping reaction , ].
Gold deposition further increases the contact angle, sincethe hydrophobic alkyl group is oriented outwards, towards the solution, due to the ammonium group having a higheraffinity to the gold and thus being attached to the gold Silanized surface
Gold modified

Our preliminary studies revealed that sensors fabricated by surface modification of the capped gold NP suffered from low sensitivity, low reproducibility and had a somewhat sluggish behaviour. We attributed this malfunction to thepoor affinity of the tetraalkylammonium bromide to the gold surface. Gradual loss of the charged moieties in aqueous Fig. 4 a SEM images of gold NP-modified EIS sensor before and b after oxygen plasma treatment of different power (time of the treatment is 10 s). b Reflectance spectroscopy of initially silanizedand gold NP-modified EIS sensors as in a J. Gun et al.
solution results in uncontrolled sensor drifts. Moreover, the Table 2 XPS elemental analysis normalized to 100% competition of the ammonium capping layer with the Silanized surface selective modifier, which is necessary for the specific sensing element, results in a low surface coverage of the gold NP bythe selective modifier. Thus, we have two seemingly contra- dicting effects: on the one hand, more potent capping agents result in lower loading of the selective modifier, and on the other hand, less effective capping agents with lower affinity to the substrate are gradually lost upon prolonged immersion in the test solutions. Our way to overcome this contradiction is to take off the capping agent altogether.
Comparison of wet chemistry methods and dry "physically" Relative error estimate =±15%a 200 W, 10 s oriented methods clearly showed the advantage of oxygenplasma treatment to remove the organic moieties. We havecarried out fundamental research in order to optimize the surface reduces the surface concentration of the Si and O performance of the oxygen plasma clean-up process. The set of elements and gives rise to new gold, nitrogen, and bromide SEM micrographs of Fig. a summarizes visually the signals. The latter are attributed to the gold NP capping agent optimization criterion of the plasma treatment. Prior to the tetraalkylammonium bromide. The sulphur content remained application of the plasma treatment one observes a corrugated approximately the same within the experimental error. The surface densely packed by the gold NP. The density and plasma treatment (200 W, 10 s) cleaned the surface of the morphology of the nanoparticles remain essentially similar sensor from the gold NP tetraoctylammonium bromide even after exposure to 100 and 200 W plasma treatments.
capping agent, as manifested by the complete removal of However, exposure of the films to 300 W and higher while the bromide and nitrogen, though traces of sulphur were still maintaining the same exposure duration (10 s) resulted in a retained, as previously reported for plasma treated SAM of distinctly lower coverage of the SiO2 surface by the gold NP.
alkyl thiolates on gold [].
Therefore, we applied 200 W plasma treatment for thesubsequent sensor fabrication. Figure provides UV–vis Examples of sensing applications reflectance measurements, which show that the plasmonpeak wavelength was indifferent to the power of the In order to demonstrate the versatility and usefulness of this plasma treatment, underscoring the fact that no nanoparticle general approach we describe four different applications coalescence/aggregation was imposed by the 200 W treat- that are based on a gold NP-modified EIS platform.
ment. This observation commensurates with Boyen's report] that no change in the gold NP size histogram was EIS-based pH sensor functionalized with gold NP observed by oxygen plasma treatment, though Boyen'streatment was conducted with a lower density of gold The basis for most enzymatic field-effect sensors con- nanoparticle coverage and at lower power intensity (50 W).
structed with Si/SiO2 structures is its sensitivity (about 30– The oxygen plasma removal of the hydrophobic capping 35 mV/decade , ]) to pH changes. Although higher agents resulted in a lower wetting angle (61°), reflecting the pH sensitivities were achieved by Ta2O5- or Si3N4-based expected small loss of hydrophobicity. The sensor fabrica- EIS sensors, SiO2-based EIS sensors remain an attractive tion was then completed by covalent or physical linkage of alternative for coupling biomolecules by means of sophis- the desirable sensing moiety to the bare gold ticated surface chemistry [–]. In the experiment, To complete the description of sensor fabrication in ConCap measurements (conducted at C=38 nF, 100 Hz) Table , the modification of the gold NP-coated sensor by in different commercial (titrisol, Merck) buffer solutions aminomethyl-appended β-cyclodextrin moiety slightly have been performed. Figure demonstrates the dynamic increases the wetting angle (despite its hydrophilic character).
behaviour of an oxygen plasma-treated gold NP-based EIS The first three steps of sensor preparation (silanization, sensor. The average pH sensitivity of was about 33 mV/pH gold modification, and plasma treatment with 200 W for 10 s) in the pH range between pH 3 and pH 10. The charac- were additionally accompanied by XPS analysis, and the teristics of those EIS sensors were almost not influenced by relevant observations are summarized in Table The the gold deposition and plasma treatment when compared determination of the elements was performed after subtrac- to a bare Si/SiO2 sensor without gold NP. This indifference tion of a Shirley background by fitting Gaussian–Lorentzian in the pH sensitivity to the presence of gold NP can be peaks to the region of interest measured with high energy explained by the absence of any weak acid moieties on the resolution. As it can be seen, gold coating of the silanized gold surface. The fundamental sensing mechanism, i.e. the

Oxygen plasma-treated gold nanoparticle-based field-effect devices transducer surface of the sensor and allows a direct and fast sensing of the reaction in case of enzyme-modified EIS Despite the high popularity of nanoparticles supported enzyme amperometric biosensors [these devices did not find EIS applications yet.
Our first exemplifying test case is the conversion of Voltage, V -1.40
glucose to gluconic acid by glucose oxidase. Since the pKa of gluconic acid is 3.6 (K=2.5×10−4) at 20 °C ], this reaction is accompanied by a change of the pH under near neutral and basic buffers (see Eq. 2GLUCONATE þ H2O2 2Hþ: Fig. 5 Typical ConCap measurement of a pH-sensitive EIS sensor modified with gold NP in Titrisol buffer solutions from pH 3 to pH 10 The immobilization of the enzyme onto the gold surface was done according to the schematic presentation in acid-base titration of the surface silanol groups remains Fig. The protocol starts with linkage of the dithio nearly not-influenced by the partial coverage of gold NP.
groups of DSP on the gold surface, and then chemicalcross-linking of protein primary amine groups through Gold NP-functionalized glucose EIS biosensor succinimide group hydrolysis and formation of the amidebonds. This immobilization scheme was first proposed by The pH sensitivity of bare EIS sensors is frequently used in Katz for the derivatization of gold surfaces ] and proved order to detect products of analytes that are converted by to be useful for the gold NP modification as well.
enzymatic catalysis yielding pH changes EIS-based In case of the NP-functionalized glucose EIS biosensor, biosensors are especially suitable for sensing oxidoreductase- the sensor stability and enzyme activity with and without originated reactions, since the oxidation (or reduction) the oxygen plasma treatment have been compared. The test activity is often accompanied by an acidification of the conditions followed the standard enzymatic assay of Sigma analyte solution (pH change). This reaction is confined to ). The spectrophotometric de- the immediate vicinity of the enzyme immobilized to the tection of the enzymatic activity of the immobilized GOx in Voltage, mV
GOx in the solution
Glucose concentration, mg/L
Fig. 6 a Typical glucose calibration curve for a gold NP-modified EIS sensor with surface-immobilized enzyme and in a GOx solution (buffer:PBS 4.5 mM, pH 7.5). b Reaction scheme of the gold NP-functionalized glucose EIS biosensor J. Gun et al.
the buffer solution was done by tracing the development of the colour (A500) by the oxidation of o-dianisidine. This dye was catalytically oxidized by hydrogen peroxide in the presence of HRP. To conduct the assay with a solid support, the EIS sensor (0.9×0.9 cm) was placed on the bottom of a 3.1 ml spectrophotometric cell with a light path of 1 cm.
Voltage, mV -0.75
Assay concentrations were 48 mM sodium acetate,0.16 mM o-dianisidine, 1.161% (w/v) glucose, and 6 U HRP (EC [Table compares the investigated assays obtained for the EIS sensors before and after plasma treatment. The plasma-treated GOx sensor exhibited abetter initial response, and its storage stability at 4 °C (PBS, 4.5 mM buffer, pH 7.5) was much superior to the 80 mV/decade
sensor that did not undergo the oxygen plasma treatment.
Figure depicts two typical calibration curves achieved from ConCap measurements for a GOx-modified gold NP 63 mV/decade
EIS sensor that is compared with a blank gold NP EIS 20 mV/decade
sensor (unmodified with GOx), where for the latter the enzymatic reaction was conducted by adding the GOx to Voltage change, mV
13 mV/decade
the solution. In this experiment, the solution contained 12 U of GOx per ml, whereas on the GOx-immobilized sensor Penicillin concentration, mM
chip there were 4.0 U of glucose oxidase (determined by Fig. 7 a Typical ConCap measurements of penicillin concentration the spectrophotometric assay in Table Both calibration with gold NP-modified EIS sensor with chemically coupled and plots follow the Michaelis–Menten dependency with physically adsorbed penicillinase, and b corresponding calibration curves in different ranges of penicillin concentration m values. However, the enzyme-modified EIS glucose biosensor reacts faster and exhibits a much superiorsensitivity compared to the blank EIS sensor that "only pH-sensitive substrates [In this study, we compared works" as a pH sensor.
two modes of penicillinase modification of gold NP-functionalized EIS sensors, chemical coupling by DSP Gold NP-functionalized penicillin EIS biosensor reagent and physical adsorption. As can be seen in Fig. both biosensors are sensitive to a wide range of penicillin The enzyme penicillinase was introduced for the first time concentrations (from 0.25 to 10 mM). Nonetheless, the for constructing a potentiometric penicillin biosensor by sensitivity of the chemically coupled penicillinase biosensor Shearer ] and for field-effect devices by Janata [Due is distinctly higher as in the case of biosensor that was to the high importance of penicillin determination for modified by physisorption, though both operate under biomedical measurements, the development of improved identical conditions (see also Fig. penicillin sensors is still a contemporary challenge [].
Penicillinase converts penicillin to penicilloic acid, which releases H+ ions ] at moderate and basic pH. The pH change is determined by the EIS sensor. Penicillinase is ahighly adhesive enzyme, and penicillin sensors can be prepared by simple adsorptive immobilization on different Pb2+ 10 -7 M
Table 3 Enzymatic activity (U/sensor) as a function of storage time Pb2+ 10-6 M
and plasma treatment for the gold NP-structured EIS biosensor Voltage, V
Pb2+ 10-5 M
Pb2+ 10-4 M
Time after sensor GOx on gold NP-modified GOx on gold NP- preparation (days) sensor after plasma Pb2+ 10-3 M
treatment (U/sensor) treatment (U/sensor) Fig. 8 Typical ConCap response of a gold NP-functionalized EIS sensor modified with aminomethyl β-cyclodextrine to sequentialaddition of lead nitrate in 18 mM PBS, pH 7.2 Oxygen plasma-treated gold nanoparticle-based field-effect devices β-cyclodextrin-modified gold NP-functionalized EIS sensor different methods and examples for the attachment of an for lead ion detection ionophore and two enzymatic sensing elements, respec-tively: Physisorption for penicillinase, DSP anchoring for As an example for a gold NP-functionalized chemical EIS glucose oxidase and penicillinase, and amine linkage for β- sensor, we demonstrate here a β-cyclodextrin-modified lead CD. Moreover, functionalized gold nanoparticles are highly ion sensor. β-cyclodextrin (β-CD) is a cyclic oligosaccharide compatible with Si chip technology and provide exceptional comprised of seven glucopyranose units Cyclodextrins versatility in chip functionalization and field-effect sensor are well known agents for including complexes formation ]. A large assortment of guest molecules, from simpleions to complicated organic molecules of ionic or neutral The authors are grateful for the technical assistance of H.-P. Bochem and A. Besmehn for the surface nature, undergoes the inclusion ligation by different CDs characterization with HRSEM and XPS and to A. Voskevich for the []. Since CDs are water-soluble, the use of these very useful discussions. J. Gun thanks the Alexander von Humboldt molecules as ionophores for sensor fabrication has to Foundation for the financial support.
involve some sort of immobilization. For this purpose, inprevious reports CDs were incorporated in a PVC gelmembrane with polymethylhydrosiloxane ] or were copolymerized with a polysiloxane gel ]. However,generally membranes lower the response time of sensors 1. Brust M, Walker M, Bethell D, Schiffrin DJ, Whyman R (1994) Synthesis of thiol-derivatised gold nanoparticles in a 2-phase and might negatively affect their sensitivity. In this liquid–liquid system. J Chem Soc Chem Commun 7:801–802 experiment, we chose to attempt a direct surface modifica- 2. Guoa S, Wang E (2007) Synthesis and electrochemical applications tion of gold NP with host ionophore molecules as an of gold nanoparticles. Anal Chim Acta 598:181–192 alternative approach for CD sensor fabrication. The sensor 3. Kharitonov AB, Shipway AN, Katz E, Willner I (1999) Gold- was constructed by the procedure described in the experi- sensitive field-effect transistors: novel assemblies for the sensing mental section. To elucidate the influence of specific of neurotransmitters. Anal Chem 18:255–260 modification of the gold NP-functionalized sensor with the 4. Katz E, Willner I (2003) Probing biomolecular interactions at ionophore on the ion caption process, we compared the conductive and semiconductive surfaces by impedance spectros-copy: routes to impedimetric immunosensors, DNA-sensors, and response of the plasma-treated gold NP sensor before and enzyme biosensors. Electroanalysis 15:913–947 after its modification with aminomodified CD.
5. Kielbassa S, Habich A, Schnaidt J, Bansmann J, Weigl F, Boyen Figure describes the dynamic response of the EIS HG, Ziemann P, Behm RJ (2006) On the morphology and stability sensor to sequential addition of lead nitrate. All measure- of Au nanoparticles on TiO2(110) prepared from micelle-stabilizedprecursors. Langmuir 22:7873–7880 ments were done in 18 mM PBS solution, pH 7.2. At the 6. Raiber K, Terfort A, Benndorf C, Krings N, Strehblow HH (2005) low concentration range (below 10−5 M) the response was Removal of self-assembled monolayers of alkanethiolates on gold rather fast and it took less than 40 s to stabilize the sensor by plasma cleaning. Surf Sci 595:56–63 signal. The calibration curve (not shown) was linear in the 7. Hesse E, Creighton JA (1999) Investigation by surface-enhanced Raman spectroscopy of the effect of oxygen and hydrogen concentration range 10−7 to 10−3 M with a slope of 8± plasmas on adsorbate-covered gold and silver island films.
0.6 mV/decade. In comparison, a blank, oxygen plasma- Langmuir 15:3545–3550 treated gold NP–EIS sensor shows less than 1 mV per 8. Gun J, Schöning MJ, Abouzar MH, Poghossian A, Katz E (2008) decade signal changes, which can be attributed to a general Field-effect nanoparticle-based glucose sensor on a chip: ampli-fication effect of co-immobilized redox species. Electroanalysis drift of the sensor rather than to a response towards lead ions. The sensitivity of the developed lead sensor is 9. Brust M, Bethell D, Schiffrin DJ, Kiely CJ (1995) Novel gold- somewhat lower compared to prior EIS lead sensors based dithiol nano-networks with nonmetallic electronic-properties. Adv on β-cyclodextrin but the linear range and the Mater 7:795–803 10. Fishelson N, Shkrob I, Lev O, Gun J, Modestov AD (2001) Studies minimum detection level are much more improved com- on charge transport in self-assembled gold dithiol films: conduc- pared to prior reports ].
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OFFICE OF PUBLIC WORKS Before the Deluge Tony Smyth, Director of Engineering Services and Chief Engineer in the Office of Public Works, talks John Walshe through the OPW's efforts to revolutionise flood risk management in Ireland via the Catchment Flood Risk Assessment and Management (CFRAM) programme. The CFRAM programme is set EU FLOODS DIRECTIvE