ORIGINAL PAPER

Fabrication and characterisation of gold nano-particlemodified polymer monoliths for flow-through catalyticreactions and their application in the reductionof hexacyanoferrate

Patrick Floris &Brendan Twamley & Pavel N. Nesterenko &

Brett Paull & Damian Connolly

Received: 9 July 2013 /Accepted: 9 October 2013 /Published online: 31 October 2013# Springer-Verlag Wien 2013

Abstract Polymer monoliths in capillary (100 μm i.d.) andpolypropylene pipette tip formats (vol: 20 μL) were modifiedwith gold nano-particles (AuNP) and subsequently used forflow-through catalytic reactions. Specifically, methacrylatemonoliths were modified with amine-reactive monomersusing a two-step photografting method and then reacted withethylenediamine to provide amine attachment sites for thesubsequent immobilisation of 4 nm, 7 nm or 16 nm AuNP.This was achieved by flushing colloidal suspensions of goldnano-particles through each aminated polymer monolithwhich resulted in a multi-point covalent attachment of goldvia the lone pair of electrons on the nitrogen of the free aminegroups. Field emission scanning electron microscopy andscanning capacitively coupled conductivity detection wasused to characterise the surface coverage of AuNP on the

monoliths. The catalytic activity of AuNP immobilised onthe polymer monoliths in both formats was then demonstratedusing the reduction of Fe(III) to Fe(II) by sodium borohydrideas a model reaction by monitoring the reduction in absorbanceof the hexacyanoferrate (ІІІ) complex at 420 nm. Catalyticactivity was significantly enhanced on monoliths modifiedwith smaller AuNP with almost complete reduction (95 %)observed when using monoliths agglomerated with 7 nmAuNPs.

Keywords Nano-particles . Agglomeratedmonolith .

Micro-reactor . Flow-through catalysis

Introduction

Great interest has surrounded metal nano-particles inrecent years due to their interesting catalytic, opticaland electrical properties. The use of gold nano-particles(AuNPs) in particular as a catalyst in electron transferreactions has been well documented [1–3] andimmobilisation of AuNP upon selected particles in themicron size range have proven to be particularlyadvantageous since they can be recovered after eachcatalytic reaction by centrifugation and can be re-usedseveral times without affecting their catalytic activity [4,5].

Monolithic materials represent another ideal solid supportfor nano-catalyst attachment because of their highpermeability and desirable flow-through properties, therebyeliminating the need for centrifugation/filtration steps that areotherwise necessary when the nano-catalyst is immobilisedupon particles. Monoliths can also be prepared in-situ within a

Electronic supplementary material The online version of this article(doi:10.1007/s00604-013-1108-2) contains supplementary material,which is available to authorized users.

P. FlorisIrish Separation Science Cluster, National Centre for SensorResearch, Dublin City University, Glasnevin, Dublin 9, Ireland

B. TwamleySchool of Chemical Sciences, Dublin City University, Glasnevin,Dublin 9, Ireland

P. N. Nesterenko : B. PaullAustralian Centre for Research on Separation Science (ACROSS),School of Chemistry, University of Tasmania, Hobart 7001, Australia

D. Connolly (*)Pharmaceutical and Molecular Biotechnology Research Centre(PMBRC), Department of Chemical and Life Sciences,Waterford Institute of Technology, Waterford, Irelande-mail: dconnolly@wit.ie

Microchim Acta (2014) 181:249–256DOI 10.1007/s00604-013-1108-2

range of housings, eliminating the technical difficultiestypically encountered when packing particles. Recentlyseveral reports have emerged describing the modification ofpolymer monoliths with selected nano-materials in an effort toincrease their active surface area for application in separationscience, which are summarised in a number of excellentreviews [6, 7]. Current methodologies involve either theencapsulation of nano-particles prior to the polymerisation[8] or their subsequent attachment upon an activatedmonolithic surface [9–13] the latter method being preferredsince a higher surface coverage is achieved.

Very few reports however, describe applications ofpolymer monoliths functionalised with nano-catalysts forflow-through catalytic reactions. Bandari et al. [14, 15]reported the immobilisation of Pd and Pt nano-particles(<2 nm in size) on a methacrylate monolith by initialcoordination of the Pd/Pt cations with di-2-pyridylamideligands immobilised upon the surface followed by theirsubsequent reduction with NaBH4. High turnovernumbers (up to 63,000) and high % yields (up to 99 %)were recorded for Suzuki and Heck coupling reactions,however aggregation and a consequent increase in the sizeof Pd nano-particles were recorded over time due to theabsence of surface-capping agents. A partial leakage(~3 %) of catalyst from the solid support was alsoreported. Other reports describe the preparation ofmethacrylate monoliths in capillary formats modified withPd-ligand complexes for flow-through Suzuki-Miyauraand Sonogashira reactions [16, 17]. High % yields ofproducts were observed, however again the partialleaching of Pd catalyst was also reported which limitedthe long-term stability of the micro-reactor. Recently ourgroup also reported the covalent immobilisation of Pd/Ptbimetallic nano-flowers on aminated capillary polymermonoliths [18]. The interconnected flow-paths in thenano-structured porous monolith (1–2 μm diameter)permitted selected redox reactions to be carried out athigh linear velocities at relatively low operatingbackpressures.

Given the ease with which polymeric monoliths can befabricated in a range of physical geometries, and the wellunderstood methods for surface modification and subsequentstable AuNP attachment, it is perhaps surprising that to ourknowledge no reports appeared in the literature to date,describing such AuNP-modified monoliths for micro-scalecatalysis. This article therefore explores this new applicationarea of AuNP-modified monoliths, clearly demonstrating theretention of catalytic properties of AuNP even after theirimmobilisation upon a monolithic surface. The flow-throughreduction of hexacyanoferrate (ІІІ) was selected for thispurpose using monoliths fabricated both in pipette tip andcapillary (100 μm i.d.) formats demonstrating applications ineither off-line or on-line modes.

Experimental

Materials and reagents

Benzophenone, 2,2-dimethoxy-2-phenylacetophenone(DAP), glycidyl methacrylate (GMA), ethylene glycoldimethacrylate (EDMA), lauryl methacrylate (LMA), 1-decanol, 1,4-butanediol, dimethylformamide (DMF), 1-propanol, ethylenediamine, hydrogen tetrachloroaurate (III),sodium citrate, sodium borohydride, tetrakis(hydroxymethyl)phosphonium chloride, potassium hexacyanoferrate, nitricacid and hydrochloric acid were all purchased from Sigma-Aldrich (Dublin, Ireland, www.sigmaaldrich.com). 4,4-Dimethyl-2-vinyl-2-oxazolin-5-one (VAL) and N-succinimidyl acrylate (NSA) were purchased from TCIEurope (Zwijndrecht, Belgium, www.tcichemicals.com/en/eu). Methanol, ethanol and acetone were of HPLC grade andpurchased from Fisher Scientific (Dublin, Ireland, www.ie.fishersci.com). All chemicals were used as received withoutfurther purification. Teflon-coated fused silica capillary(100 μm i.d.) was supplied by Composite Metal Services(Shipley, England, www.cmscientific.com). The 20 μLpolypropylene pipette-tips for in-situ monolith fabricationwere from Brand (Wertheim, Germany, www.brand.de).

Instrumentation

Photo-polymerisation and photo-grafting were carried outusing a Spectrolinker XL-1000 UV Crosslinker at 254 nm(Spectronics Corp., Westbury, NY, USA, www.spectroline.com). The balance used was a Sartorius Extend (Sartorius,Goettingen, Germany, www.sartorius.com). A KD Scientificsyringe pump (KDS-100-CE, KD Scientific Inc., Holliston,MA, USA, www.kdscientific.com) was used for all washingand functionalisation steps of monoliths in both pipette tip andcapillary format, while a Model K120 Knauer pump (Knauer,Berlin, Germany, www.knauer.net) was used for themodification of monoliths with AuNP in capillary formatonly. Field emission scanning electron microscopy (FE-SEM) and energy dispersive X-ray spectroscopy (EDX) wasperformed using a Hitachi S-5500 instrument (Hitachi,Maidenhead, UK, www.hitachi-hta.com) equipped with anOxford Instruments PentaFETx3 detector with the INCAMicroanalysis Suite (Oxford Instruments, Oxfordshire, UK).UV spectra were obtained using a Lambda 900 UV–visspectrophotometer (Perkin Elmer, Waltham, MA, USA,www.perkinelmer.com), while for measurements in flow-through mode a Dionex Ultimate 3000 system equipped witha 3 nL flow cell was used (Sunnyvale, CA, USA, www.thermofisher.com/global/en/products/dionex). A TraceDeccapacitively coupled contactless conductivity (C4D) detector(Innovative Sensor Technologies, GmbH, Innsbruck, Austria,www.ist-ag.com) was used for the characterisation of

250 P. Floris et al.

monoliths in capillary formats. A Zetasizer Nano ZS particleanalyser (Malvern Instruments, Westborough, MA, USA,www.malvern.com) was used for obtaining particle size andzeta potential data of the colloidal suspensions. A QuorumEmitech K575x (Quorum, Kent, UK, www.quorumtech.com)sputter coater was used to apply a 45 nm layer of chromiumprior to EDX analysis.

Synthesis of colloidal AuNP suspensions

Citrate stabilised AuNP (16 nm), were prepared according tothe procedure outlined by Frens [19]. Similarly, 7 nm citratestabilised AuNP were prepared using NaBH4 as the reducingagent as described by He et al. [20]. Finally THPC stabilisedAuNPs (4 nm) were also prepared following a procedurereported by Duff et al. [21]. Detailed procedures are providedin the Electronic Supplementary Material (ESM).

Preparation of AuNP-modified monoliths in pipette-tipand capillary formats

Polymer monoliths in pipette tip formats and capillary formatswere fabricated, grafted, aminated and modified with goldnanoparticles as described in the accompanying ESM. Areaction schematic for the grafting and subsequent aminationof the monoliths is shown in Figure S1 and the monomer/porogen composition of all monoliths is described in Table S1.

Catalytic reduction of Fe(ІІІ) to Fe(ІІ)

Standard redox chemistry was used to evaluate the catalyticproperties of the immobilised AuNPs in either an off-line oron-line set up. A mixture of 4 mMK3[Fe(CN)6] and 0.6 mMNaBH4 was prepared in 0.085M NaCl, pH 11.5. This solutionwas pumped at 0.25 mL.hr−1 for 1 h through the EDMA-Tip-16 and a blank EDMA-Tip for comparative purposes. Thecollected effluent was subjected to UV–vis analysis at420 nm. Similarly the experiment was performed on-line byinjecting 100 nL amounts of selected concentrations ofK3[Fe(CN)6] on AuNP functionalised capillary monoliths(LMA-GMA-16, LMA-GMA-7 and LMA-GMA-4) whilepumping water at 1 μL.min−1 or 3 μL.min−1. The progressof the reaction was monitored relative to LMA-GMA-Blk at adetector wavelength of 420 nm using a 3 nL flow-cell.

Results and discussion

Characterisation of AuNPs

The size and shape of AuNP are known to have a significanteffect upon their catalytic activity [22] and therefore in thisstudy, three sizes of AuNP were prepared prior to monolith

fabrication. Predictably, the nanoparticles stabilised by citrate(16 nm and 7 nm) had a strong negative surface charge leadingto a stable colloidal suspension whereas in our hands thesmaller AuNP stabilised by THPC exhibited somewhat poorercolloidal stability. The absorbance maxima due to surfaceplasmon resonance increased as particle size increased asshown in Figure S2. The size, surface zeta potential, λmax

and concentration for each type of AuNP is detailed inTable S2 as determined using a method described by Liuet al. [23].

Preparation of polymer monoliths in pipette tips

Polymer monoliths were used as solid supports for theimmobilisation of AuNP because of their ideal porousstructure making them ideally suited to flow-throughapplications, their ease of preparation via facile in situphotoinitiated fabrication methods and their readilycustomisable chemical properties. In this work we elected tomaximise the number of reactive functional groups present atthe monolith surface using photografting methods firstdescribed by Stachowiak et al. [24]. Vinyl azlactone (VAL)was grafted in favour of GMA since poly(VAL) can be readilyaminated at room temperature. Figure 1a, b illustrates theintimate contact between the monolith and the polypropylenepipette-tip housing whereas Fig. 1c, d demonstrate that thecoverage of AuNP was radially homogeneous with someevidence of nano-particle aggregation as revealed by clustersof white dots on the surface of the monolith globules.

Energy-dispersive X-ray spectroscopy (EDX) was alsoused to confirm qualitatively the presence of 6.4±1.7 (n =3)atom% gold on the monolithic tip (see ESM, Figure S3 andFigure S4) which compares well with results of Xu et al. whoreported 5.5±0.6 atom% and 15.4±1.6 atom% dependingupon the protocol adopted [13].

Catalytic reduction of Fe(ІІІ) to Fe(ІІ) via AuNP-modifiedmonolith in pipette-tip format

The catalytic reduction of ferrocyanide (ІІІ) by NaBH4 wasselected as a model reaction for demonstrating the catalyticproperties of the AuNP agglomerated monoliths. The redoxmechanism at the AuNP surface is similar to that discussed byFloris et al. [18]. During operation, the most obvious indicatorof AuNP-mediated reduction was a change in colour of thehexacyanoferrate(ІІІ) solution from yellow to colourlessduring passage through the EDMA-Tip-16 monolith. UV–vis analysis of collected effluent revealed a significantdecrease in absorbance at 420 nm. An overlay of UV–visspectra is shown in ESM (Figure S5) which allowscomparison between EDMA-Tip-16 and EDAM-Tip-Blk.The redox reaction occurred considerably faster over a periodof 1 h on the EDMA-Tip-16 monolith (Figure S5c) compared

Fabrication and characterisation of gold nano-particle modified polymer monoliths 251

with EDMA-Tip-Blk (Figure S5b) which is in agreement withpreviously reported observations that the catalysed reactionoccurs 4×104 times faster in the presence of a catalyst [25].

Scanning contactless conductivity characterisationof AuNP-modified capillary monoliths

Recently, scanning capacitively coupled contactlessconductivity detection (sC4D) has emerged as a potentialnon-invasive characterisation tool for capillary monoliths.Using a commercial C4D detector in “scanning” mode, theaxial homogeneity of semi-permanent surfactant coatings orpolymer grafts on capillary monoliths have been indirectlyvisualised in a non-invasive and non-destructive manner asdescribed recently in an excellent review by Connolly et al.[26]. More recently, Currivan et al. reported for the first timethe use of sC4D to visualise the axial distribution of AuNPalong an aminated segment of polymer monolith [27].Building upon that report, sC4D was used herein to examinethe axial distribution of AuNP along the monolith substrate,without the need destroy the monolith by cutting into separatecross-sections for analysis.

The LMA-GMA-16 monolith was subjected to sC4Dcharacterisation before and after amination as well as duringand after the immobilisation of AuNP. Figure 2a illustrates aclear increase (16 %) in mean conductive response afteramination; the distribution of amine groups was axiallyhomogenous along the measurable length of monolith. Duringthe subsequent AuNP coating step, the gradual progress ofAuNP along the monolith could be easily “visualised” usingsC4D profiling methods allowing the axial distribution of

16 nm nano-particles along the monolith bed to be evaluated.Figure 2b and c show that the conductive response of theAuNP-modified segment (1 cm and 5 cm respectively)decreased significantly relative to the remainder of themonolith (to within 3.1 % and 2.1 % of the bare ungraftedmonolith for Figure 2b and c respectively).

This observation is in close agreement with those reportedby Currivan et al. [27] and is presumably due to a suppressionof the conductive response when the negatively charged nano-particles attach to the pendant primary amine groups.Interestingly, the distribution of AuNP along the coated regionof monolith (particularly noticeable in Fig. 2c) was veryhomogenous (129±1.4 mV), and the conductive responserose sharply within 2 mm as the C4D cell traversed theboundary between the AuNP-coated and unmodified regionsof the monolith. Figure 2d appears to illustrate a poorhomogeneity of coverage, but this is more likely due toinadequate rinsing with water to remove citrate anions priorto C4D profiling, particularly given the significantly highererror bars (125±2.7 mV). A similar sC4D profilingexperiment was performed on a separate LMA-GMAmonolith after coating with smaller 7 nm AuNPs. Thecorresponding data is presented in the ESM as Figure S6.

Characterisation of AuNP coverage on monoliths by FE-SEM

While sC4D profiling permitted the evaluation of the axialdistribution of nano-particles along the monolith field-emission SEM was used to directly visualise the coverage ofAuNP on the surface and to examine the effect of theamination protocol (stopped flow or continuous) as well as

Fig. 1 Optical photographs andFE-SEM images of EDMA-Tip-Blk (a , b) and EDMA-Tip-16 (c ,d). FE-SEMmagnification: 5,000(b) and 50,000 (d)

252 P. Floris et al.

the effect of grafting either an amine-reactive methacrylate(GMA) or an acrylate (NSA). Figure 3a, b and c, d representthe LMA-GMA-16 monolith which had been aminated usingthe “stopped flow” and the “continuous flow” methodsrespectively. The stopped flow method resulted incomparatively sparse coverage of AuNP (115/500 nm2) withobvious signs of nano-particle aggregation (dimers, trimersand larger clusters) whereas the continuous flow methodresulted in a clear increase in AuNP coverage (96 %) to 225/500 nm2 with a more homogeneous distribution across thesurface.

Efforts to further improve the surface density of AuNPinvolved the grafting of an amine-reactive acrylate monomer(NSA) instead of a methacrylate monomer based upon reportsby Yang et al. [28] who reported that acrylate monomers havehigher photografting reactivities relative to methacrylatemonomers due to the presence of allylic hydrogen atoms inthe monomer. Therefore the use of NSAwas investigated as analternative graftingmonomer to glycidyl methacrylate, using a“continuous flow” amination at ambient temperature in thesubsequent modification step. Figure 3e, f shows that graftingan acrylate monomer resulted in a further increase in coverageto 272/500 nm2, representing a 21 % increase over the GMA-grafted monolith in Fig. 3c, d, presumably due to improvedgrafting efficiency relative to GMA. Although the AuNPsurface coverage was maximised with the use of NSA,nevertheless for further studies the GMA chemistry wasadopted since pendant NHS esters (after grafting of NSA)are known to be subject to unwanted hydrolysis (half-life of1 h at pH 8.0 and 25 °C [29] potentially leading to lower

repeatability of AuNP coverage in this work relative to themore robust epoxy chemistry of GMA-grafted monoliths.

Fe-SEM imaging of 7 nm and 4 nm AuNP on polymermonoliths was a considerable challenge due to the resolutionlimitations of the instrumentation used in this study(particularly for nano-particles<10 nm on a non-conductingpolymer monolith). Nevertheless, images presented in theESM (Figure S7) show that coverage was less homogeneousthan for 16 nm particles, with large patchy aggregates orplaques evident, particularly for the 4 nm THPC-stabilisednano-particles.

Catalytic reduction of Fe(ІІІ) to Fe(ІІ) via AuNP-modifiedmonoliths in capillary formats

The monitoring of AuNP-catalysed redox reactions via apipette-tip monolith required the collection of fractions forsubsequent UV analysis which did not permit real-timeanalysis of the reaction products in any practical way.Therefore, the fabrication of AuNP-modified monoliths incapillary formats more readily facilitated a study into theeffect of operational flow rate, concentration of reagents andsize of immobilised AuNPs upon catalytic activity using aflow-through UV detector.

Selected concentrations of K3[Fe(CN)6] were injected via a100 nL loop while pumping water at either 1 μL.min−1 or3 μL.min−1. A 3 nL flow cell at the end of the monolithallowed detection of product which manifested as a decreasein absorbance at 420 nm relative to a blank aminatedmonolith. (Note: 4 mMK3[Fe(CN)6] was used in Fig. 4 such

Fig. 2 sC4D profiles of LMA-GMA-16 before coating with16 nm AuNP (a) and after a 1 cmsection (b) a 5 cm section (c) anda 7 cm section (d) were coated.Blank monolith (diamond),grafted/aminated monolith (whitesquare), AuNP-coated monolith(triangle). Shaded zones of thefigures indicate regions whichwere AuNP-modified, at thatparticular time-point

Fabrication and characterisation of gold nano-particle modified polymer monoliths 253

that the difference in absorbance for LMA-GMA-4 and LMA-GMA-7 could readily be visualised as discussed later).Figure 4 shows results of the effect of flow rate upon peakshape by comparison of the peak width at 50 % height at1 μL.min-1 (a) and 3 μL.min−1 (b) (equivalent to linearvelocities of 5 mm.sec−1 and 15 mm.sec−1 for each respectiveflow rate). Peak width for the LMA-GMA-Blk monolithdecreased from 1.1 min to 0.18 min when a higher linearvelocity was used and residence times of K3[Fe(CN)6] withinthe micro-reactor was calculated to be 75 s and 25 s for the lowand high linear velocities used in the study. The effect of bothresidence time and the size of immobilised AuNPs uponcatalytic activity was studied, using a calibration curve of 0–17.5 mMK3[Fe(CN)6]. From a practical standpoint, sincepeaks were still not perfectly Gaussian, peak height was usedrather than peak area for quantitation of product formed.Figure 4a and b show that iron (III) hexacyanoferrateconcentration steadily decreased relative to a LMA-GMA-Blk for monoliths modified with progressively smaller AuNP.

For example as shown in Table 1, at 5 mm.sec−1, theconcentration decreased by 57 % for LMA-GMA-16, 87 %for LMA-GMA-4 and 95 % for LMA-GMA-7.

Increasing the linear velocity to 15 mm.sec−1 resulted in nosignificant reduction in catalytic activity. (Note: higher linearvelocities were not possible with this system due to pressurelimitations. Higher flow rates would necessitate capillarymonolithic micro-reactors of a larger diameter: monoliths upto 250 μm i.d. have been successfully produced using similartechniques described here).

This confirms previous reports in the literature whichdemonstrated that particles of smaller size can be catalyticallymore efficient due to their higher surface area [30]. Againstthis trend however, LMA-GMA-4 which was modified with4 nm THPC stabilised AuNPs was found to operate slightlyless efficiently than LMA-GMA-7. Catalytic activity is knownto be dependent on the concentration of catalyst [25], thenature of the surface ligands themselves as well as their spatialarrangement/surface density [30] and the presence of different

Fig. 3 FE-SEM images of LMA-GMA-16 which had beenaminated in stopped flow mode,(a , b) and continuous mode (c ,d) and monolith LMA-NSA-16which was aminated incontinuous mode (e , f).Magnification: 50,000 (a , c , e)and 100,000 (b , d , f)

254 P. Floris et al.

surface crystallographic planes on the nano-catalyst [31]. It ispossible therefore that the shape of the 4 nm THPC-stabilisednano-particles, in combination with the use of THPC asstabilising ligand rather than citrate (as for LMA-GMA-16and LMA-GMA-7) had a negative effect upon catalyticactivity. Finally, the poor colloidal stability (zeta potential:

0.6±2.8 mV) may have lead to aggregation on the monolithsurface resulting in larger nano-particle clusters as shown inFigure S7 in the ESM, thus reducing the expected catalyticactivity.

The redox reaction selected in this work to demonstrate theutility of the developed AuNP-modified polymer monolithswas carried out under relatively mild reaction conditions andarguably has limited applicability. Therefore it is pertinent toconsider whether the same micro-reactor could potentially beused for more useful reactions involving harsher conditions. Avery important reaction in organic chemistry is the gold-catalysed Suzuki coupling reaction in which carbon-carbonbonds are formed between an aryl halide and phenylboronicacid to produce biaryl in solution, involving temperatures upto 80 °C for 4 h in the presence of a strong base [32]. Althoughnot demonstrated directly in this presented work, our researchgroup have nevertheless previously demonstrated in a relatedpaper that AuNP cannot be detached from an aminatedmonolith despite flushing 4M mercaptoethanol for at least60 mins followed by a water flush at 80 °C for prolongedperiods [27]. This result is indicative of the future potential forAuNP-modified monoliths to be applied in a broader range ofapplications, which will be the subject of future work withinour research group. In our hands the AuNP-modifiedmonoliths could be re-used several times and were stable forat least 6 weeks of continuous use which included periodswhere they were stored dry, with no obvious signs of nano-particle loss (which would manifest as a reduction in redcolouration along the monolith length).

Conclusions

Polymer monoliths modified with a dense coverage of AuNPof selected sizes have been shown to have utility as flow-through catalytic micro-reactors. The robust nature of AuNPattachment to the grafted polymer substrate via multi-pointinteractions between each nano-particle and the aminatedsurface will allow the scope of application to be considerablybroadened to more interesting reactions (Suzuki couplingreactions [32]) which require significantly harsher operatingconditions than those used in the simple Fe (III)/Fe (II) redoxconversion demonstrated here. Furthermore, we propose thatthe micro-reactor described here also has potential applicationin areas outside of classical organic micro-scale synthesis,namely post-column reactions in liquid chromatography. Forexample, it is well known that gold nano-particles act as acatalyst to enhance luminol/H2O2 chemiluminescence whichis a common post-column reaction used in HPLC andsummarised in a recent review by Wu et al. [33]. In a typicalset-up three reagents are combined post-column, HAuCl4,luminol and H2O2 resulting in the in-situ generation of AuNPdue to the strong reducing effect of the carbonate/bicarbonate

Fig. 4 Reduction of iron (ІІІ) to iron (ІІ) on AuNP-modified capillarymonoliths at (a) 5 mm.sec-1 and (b) 15mm.sec-1. Absorbance at 420 nmis plotted against time for (i) LMA- GMA-Blk, (ii) LMA-GMA-16, (iii)LMA-GMA-4 and (iv) LMA-GMA-7. Monolith dimensions: 100μm×130 mm. Iron (III) solution: 4 mM K3[Fe(CN)6] and 0.6 mM NaBH4 in0.085 M NaCl, pH 11.5

Table 1 Reduction of hexacyanoferrate (ІІІ) on AuNP modified LMA-GMA capillary monoliths

Concentration(mM)(n=3)

% Fe(ІІІ)reduced toFe(ІІ)

Average rateof reaction (mM s−1)(M-M0)/(t-t0)

15 mm s−1

LMA-GMA-Blk 4.86±0.09 –

LMA-GMA-16 2.07±0.26 57 % −0.08LMA-GMA-4 0.49±0.13 90 % −0.13LMA-GMA-7 0.23±0.05 95 % −0.14

5 mm s−1

LMA-GMA-Blk 4.41±0.04 –

LMA-GMA-16 1.89±0.09 57 % −0.03LMA-GMA-4 0.59±0.00 87 % −0.04LMA-GMA-7 0.23±0.06 95 % −0.04

Fabrication and characterisation of gold nano-particle modified polymer monoliths 255

luminol buffer system. Analytes are detected due to eithertheir enhancing or inhibitory effect upon the resultingchemiluminescence. The AuNP-modified micro-reactordescribed in this work has potential to eliminate therequirement for an individual supply of HAuCl4, simplifyingthe experimental set-up and providing a catalytic surfacewhich would also ensure good post-column reagent mixingdue to the presence of multiple tortuous flow paths in thepolymer monolith.

Acknowledgments The authors would like to thank ScienceFoundation Ireland (Grant number 08/SRC/B1412) for research fundingunder the Strategic Research Cluster programme and also for equipmentfunding (Grant. Number 03/IN.3/1361/EC07).

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