Hierarchically-Ordered Electroactive Silica-Polyaniline Nanohybrid: A Novel Material with Versatile Properties
MANESH KALAYIL MANIAN,1GOPALAN ANANTHA IYENGAR,1,2KWANG-PILL LEE,1,2NAM HEE KIM,1
KOMATHI SHANMUGASUNDARAM,1SEONG HO KANG3
1Department of Chemistry Graduate School,Kyungpook National University,Daegu702-701,South Korea
2Department of Nanoscience and Nanotechnology,Kyungpook National University,Daegu702-701,South Korea
3Department of Applied Chemistry,College of Applied Science,Kyung Hee University,Yongin-si,Gyeonggi-do446-701, South Korea
Received1December2009;accepted11July2010
DOI:10.1002/pola.24245
Published online in Wiley Online Library(https://www.doczj.com/doc/692847681.html,).
ABSTRACT:Poly(trimethoxy silylpropylaniline),a nanoporous (pore diameter of2.4nm),electroactive(stable reversible redox characteristics),electrochromic(yellow atà0.10V,blue green att0.50V,and dark green att0.70V),and pH-sensitive, silica–polyaniline(PANI)hybrid material(designated as KGM-1) has been synthesized in powder form by a simple one-pot chemical synthesis as well as a‘‘thin nanolayered film’’by cyclic voltammetry.High-resolution transmission image of KGM-1informs that the particles are spherical,with diameters in the range of0.5–1.5l m.X-ray diffraction pattern of pristine KGM-1confirms the combined presence of ordered silica net-work and PANI chains.The surface area of calcined KGM-1is 40m2/g($15times higher than KGM-1),and the average pore size is 2.4nm.The N2adsorption features also inform that PANI is present as a uniform layer within the pores of silica and because of that the silica pores are not completely blocked.The reversible redox transitions in PANI units and nanoporosity of KGM-1are effectively used for the electro-driven loading/release of DNA or adenosine50-triphosphate. V C2010Wiley Periodicals,Inc.J Polym Sci Part A:Polym Chem 48:4537–4546,2010
KEYWORDS:electroactive;nanocomposites;nanoporous;nano-technology;polyaniline;silicas
INTRODUCTION Organic–inorganic hybrid materials have attracted research interest in fundamental and applied fields.1–3 Hybrid nanomaterials,consisted of a conducting polymer(CP) within the porous silica combine the special properties of an inor-ganic matrix(chemical stability,morphology,tunable porosity,op-tical transport,and mechanical strength)with electrical and/or electronic properties of CPs.These materials have a wide range of applications,such as sensors,4,5microelectronics,or photonics.6–9 Since Kresge et al.10reported the synthesis of M41S family of sili-cates(like MCM-41),a variety of ordered mesoporous silica-based materials have become available to host molecules of various sizes (small organic to polymer),shapes,and functionalities.The first report on the preparation of silica–CP hybrid material has been made by Wu and Bein11using in situ polymerization of aniline within the channels of porous aluminum silicate.Studies have then been extended for the inclusion of CP within the channels of mesoporous materials.12–15Silica–CP hybrids have been synthe-sized by polymerization of CP on the surface of silica,16in situ hy-drolysis and condensation of tetraethyl orthosilicate in solution of CP or on the surface of CP.17Functional nanohybrid materials hav-ing hierarchically ordered chains of CP within the nanoporous inorganic matrix are expected to find diversified applications in fields such as drug delivery,sensors,nanodevices,and so forth.10,18
As an important material in the family of CPs,polyaniline(PANI) has attracted intensive research because of its promising electri-cal,electrochemical,and optical properties.19Reports are avail-able on the encapsulation of PANI or PANI derivatives within pores of MCM-41.20–24Chemical anchoring of silica nanopar-ticles onto PANI chains has been reported through electrocopoly-merization of aniline.25Electrochemical reactive insertion has been used for the preparation of PANI–silica composite.26Nano-composites of mesoporous MCM-41and PANI have been pre-pared.27CPs are suited for the programmed uptake or release of biomolecules(drugs,DNA,adenosine50-triphosphate[ATP], viruses,etc.).We envisage that an effective electro-driven drug uptake or release is feasible for a nanoarchitectured CP hosted within a porous,rigid,and inorganic frame like silica network. The preparation of silica–PANI nanohybrids is hampered by few challenges and obstacles,in terms of extent of encapsu-lation of a CP within the silica pores,structural alignment within the channels,and periodicity of functionalities.The encapsulation of a lower extent of PANI chains and lack of
Additional Supporting Information may be found in the online version of this article.Correspondence to:K.-P.Lee(E-mail:kplee@knu.ac.kr) Journal of Polymer Science:Part A:Polymer Chemistry,Vol.48,4537–4546(2010)V C2010Wiley Periodicals,Inc.
periodicity of arrangements of PANI chains within the silica channel have been earlier notified.28A simple method for the preparation of silica–PANI hybrid with the possibility to obtain as thin film is highly warranted.
Recently,self-assembly has emerged as a powerful technique for controlling structure and properties of ensembles of or-ganic/inorganic particles.In recent reviews,Ariga et al. described the details of two-dimensional nano architectonics based on self-assembly at an interface.29,30Several silica-based functional nanomaterials have been prepared by self-assembly approach.31A versatile PANI-catalyzed silicification route has been established for synthesizing functional nanomaterials.32A novel and simple strategy has been established for the prepara-tion of fluorescent nanoparticles by self-assembly of conjugated polymer onto the core-shell Ag@SiO2.33A multifunctional nano-composite incorporating cadmium selinide quantum dots and iron oxide(Fe3O4)nanoparticles within silica nanotube matrix has been prepared based on self-assembly of preformed func-tional nanoparticles.34
Herein,we report the facile one-pot synthesis of poly N-[3-(tri-methoxy silyl)propyl]aniline,a silica–PANI hybrid material (designated as KGM-1).KGM-1has several special features, such as nanoporosity,periodically arranged PANI units within silica nanochannels,electroactivity,electrochromic behavior, and drug-loading capabilities.
EXPERIMENTAL
Materials
N-[3-(Trimethoxy silyl)propyl]aniline(TMSPA),ammonium persulfate(APS),and b-naphthalene sulfonic acid(b-NSA) were purchased form Aldrich(St.Louis,MO).
Prepartion of KGM-1(Chemical Method)
A solution of TMSPA(50m M)was prepared in b-NSA(50 m M),and the solution was stirred for2h.Ten milliliters of 0.5M APS was added dropwise to TMSPA solution with con-stant stirring at5 C.A dark green-colored precipitate(poly N-[3-(trimethoxy silyl)propyl]aniline doped b-NSA,KGM-1) was obtained.The precipitate was washed with distilled water and dried in vacuum oven at60 C.
KGM-1was treated with an aqueous solution of ammonia(pH %9)under stirring for6–8h to obtain neutral(dedoped)form of KGM-1.The reaction mixture was filtered and washed with distilled water.The blue powder(neutralized KGM-1)collected by filtration was further purified by refluxing in methyl alcohol. The KGM-1neutralized was dried at60–80 C for8h.KGM-1 (neutralized)was calcined at540 C,and a white powder (KGM-1,calcined)was collected.
Prepartion of Conventional PANI
A solution of aniline(50m M)was prepared in b-NSA(50m M). Ten milliliters of0.5M APS solution was added dropwise to ani-line solution with constant stirring at5 C.The precipitate,con-ventional PANI,was washed with distilled water and dried in vacuum oven at60 C.Preparation of KGM-1Thin Film
(Electrochemical Method)
Cyclic voltammetry measurements were made by using EG and G PAR283Potentiostat/Galvanostat.A solution of TMSPA(50 m M)in b-NSA was electropolymerized in a three-electrode cell assembly consisting of indium tin oxide(ITO)-coated glass plate, Ag/AgCl(sat),and platinum wire as working,reference,and coun-ter electrodes,respectively,by reversibly cycling the potentials in the range between0.0V andt1.0V.A green-colored film(KGM-1)was deposited on the surface of ITO electrode.Cyclic voltam-mogram(CV)of KGM-1film was recorded in1M NSA at scan rate of100mV/s to evaluate its electroactivity.The thickness of KGM-1film was evaluated by graphical integration of area under the cyclic voltammetric curves.
Characterization
Morphology and Structure of KGM-1(Powder)
The particle morphology of KGM-1powder was investigated using high-resolution transmission electron microscope (HRTEM;Jeol,JEM-ARM1300S)with a field emission electron gun operated at1250kV.X-ray diffraction(XRD)patterns of the samples were collected using a D8-Advanced Bruker AXS dif-fractometer using Cu-a K radiation.N2adsorption–desorption isotherms(Brunauer–Emmett–Teller)were recorded using a Quantachrome Autosorb-1with nitrogen as the adsorbate at77 K.The samples were degassed for2h at200 C in vacuum before measurement.UV-vis spectra were recorded using Shi-madzu UV-2101spectrophotometer at room temperature in the range of300–800nm.Surface area and pore sizes were deter-mined using Quantachrome,Nova2000,Germany.Pore sizes were obtained from adsorption branches of N2adsorption–de-sorption isotherms(Brunauer–Emmett–Teller).Fourier trans-form infrared spectrometer(FTIR;Jasco,IR-610)was used to record the spectra using KBr pellets.Thermogravimetric analy-sis(TGA;Seiko,TMA120,Japan)of the samples was made at a heating rate of10 C/min under a nitrogen atmosphere over a temperature range of30–800 C.Conductivity was determined at room temperature by a standard four-probe van der Pauw method using Accent,HL5500,UK.The powders was pressed into6-mm pellet at6MPa and used for conductivity measure-ment.Elemental analyzer(Carlo,Erba1106)was used to deter-mine the elemental compositions.
Electrochromism and Spectroelectrochemistry
of KGM-1Film
KGM-1film of desired thickness was deposited on ITO (working electrode)as detailed above using cyclic voltamme-try.The experiments were done in quartz cuvette of1cm path length by assembling cell with ITO,Ag/AgCl,and plati-num wire as working,reference,and counter electrodes, respectively.UV-vis spectra were recorded concomitantly by applying potentials in the rangeà0.1V to0.7V.
The Home-Made Total Internal Reflection
Fluorescence Microscopy
The basic experimental set-up of the home-made total inter-nal reflection fluorescence microscope(TIRFM)for the direct observations of individual single DNA molecules on the sub-strates was similar to that described elsewhere.35,36Briefly,
JOURNAL OF POLYMER SCIENCE:PART A:POLYMER CHEMISTRY DOI10.1002/POLA
a TIRFM with a nanometer precision positioning controller (Digital Bio Technology Co.,Ltd.,Seoul,South Korea)was constructed using a transmitted all-side polished dove prism (BK7,15mm;63mm;15mm,n ?1.522,Korea Electro-Optics Co.,Ltd.,South Korea).The TIRFM optics was incor-porated into an upright Olympus BX51microscope (Olympus Optical Co.,Ltd.,Shinjuku-Ku,Tokyo,Japan)along with a DIC slider (U-DICT,Olympus).An oil-type 100?objective lens (Olympus UPLFL 100?/1.3N.A.,W.D.0.1)was used.A CCD camera (Cascade 512B,Photometrics,Tucson,AZ)was mounted on the top of the microscope.The CCD exposure time was 100ms.A 488-nm notch filter (Korea Electro Optics,South Korea)and a 520-nm barrier filter (Olympus Optical Co.,Ltd.)was used to carry out the TIRFM study.An argon ion laser at 488nm with a 40-mW output (model 532-LAP-431-220;Melles Griot,Irvin,CA)was used as the excitation source for TIRFM.
A film of KGM-1($400nm thickness)was coated onto the sur-face of ITO and placed on an all-side polished dove prism.The KGM-1($400nm thickness)-coated ITO plate and prism were index-matched with a drop of immersion oil (ImmersolTM 518F,Zeiss,n ?1.518).The laser beam was then directed through the prism toward the protein chip/media interface.The angle of incidence,i ,was slightly greater than 72 .To
reduce photobleaching,a Uniblitz mechanical shutter (model LS3Z2;Vincent Associates,Rochester,NY)was used to block the laser beam when the camera was off.MetaMorph 7.0soft-ware (Universal Imaging Co.,Downing Town,PA)was used for image collection and data processing.Potential (à0.4or t0.8V)was applied to KGM-1film-coated ITO electrode ($400nm thickness)using a high-voltage power supply (E3612A,Agilent).
RESULTS AND DISCUSSION
Synthesis of KGM-1involves sequential reactions:oxidation of aniline units in TMSPA,hydrolysis of silanol units,and polymerization (Scheme 1).The one-pot synthesis is simple compared with conventional preparation of silica–CP hybrid materials.In the conventional methods,initial functionaliza-tion of silica walls and postsynthetic reactions were required.In such cases,the extent of loading of the poly-meric chains solely depended on the functionalization step.However,the present one-pot methodology avoids the tedi-ousness in the multistep procedure for the preparation of silica–PANI hybrid (Scheme 1).Generally,MCM type honey comb structures are formed by the sol–gel synthesis involv-ing organosilicate precursors through self-assembly of amphiphilic surfactants.However,preparation of KGM-1
does
SCHEME 1Mechanism of KGM-1formation.(A)Spherical micelles of NSA-TMSPA self-assembly.(B)Simultaneous polymerization and condensation.(C)Polyaniline within the nanochannel of silica.
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not involve a long-chain surfactant.In the present method, b-NSA,having a hydrophobic naphthalene unit and a sulfo-nate head group,was used to prepare KGM-1.NSA plays the roles of a micelle-directing agent for the formation of KGM-1 and a dopant for PANI chains.
The mechanism of formation of KGM-1involves sequential stages;self-assembly of TMSPA molecules over the spherical micelles of NSA(through electrostatic interactions between the-NH2group in TMSPA and SO3àgroup in NSA),oxidation of-NH2groups,polymerization of aniline units,hydrolysis of -OMe groups,and silicate formation through condensation of -OH groups(Scheme1).The aromatic amine groups in TMSPA catalyze the hydrolysis.37The mechanism of forma-tion of KGM-1is similar to the biological production of silica.38Thus,KGM-1contains the silicate network as well as the covalently linked and periodically ordered PANI chains (Scheme1).KGM-1can be converted into neutralized and calcined materials as follows.The as-prepared KGM-1is a b-NSA-doped(green colored)material.Deprotonation of PANI chains in KGM-1was performed by treatment with aqueous NH3.KGM-1transformed into neutralized(blue colored)ma-terial.Neutralization is accompanied by dedoping of PANI chains through removal of protons.The neutralized KGM-1 was subjected to calcination to obtain calcined KGM-1.PANI chains within the silica pores were decomposed by calcina-tion.KGM-1,KGM-1(neutralized),and KGM-1(calcined) were characterized independently.
XRD pattern of pristine KGM-1confirms the combined pres-ence of ordered silica network and PANI chains.Small-angle XRD pattern of KGM-1[Fig.1(A)]exhibits the characteristic diffraction peaks assignable to a hexagonal symmetry of meso-porous structures.A strong reflection peak d(100)(2h?2.9) with three small peaks d(110)(2h?3.3),d(200)(2h?5.9),and d(210)(2h?6.6)indicates regular,hexagonal array of uniform channels of mesoporous structure with long-range order and good textural uniformity.The corresponding d spacing values for hkl planes(100),(110),(200),and(210)are29.47,26.59, 14.78,and13.28?,respectively.It should be remembered that the XRD patterns reported by Kresge et al.10for MCM-type mesoporous materials show peaks with d-spacing values at2h ?2.24 (39.8),3.92 (22.9),4.45 (19.8),and5.89 (14.9)for 100,110,200,and210planes,respectively.The comparision between the diffraction peak intensity and peak width informs that the scattering domain size and the pore structures are uniform and ordered as similar to MCM type materials reported earlier.XRD pattern of KGM-1[Fig.1(A),inset]also displays a peak around26 and a shoulder around30 that are characteristics of amorphous PANI.The intensities of peaks and pattern of higher-order diffraction peaks are slightly differ-ent compared with ordered silica structure.The difference in XRD pattern may be attributed to the presence of PANI chains within the silica pores.Importantly,PANI chains are covalently interlinked(grafted)to the silica walls.The interplanar spacing (d100)of KGM-1(29.4?)is relatively higher than that of MCM-41prepared with surfactants having an aliphatic chain length longer than10.39The crystalline domain size of KGM-1 is lower than the conventional PANI.40HRTEM image of KGM-1[Fig.1(B)]informs that the par-ticles are spherical,with diameters in the range of0.5–1.5l m[Fig.1(B)(a)].The existence of amorphous PANI in the pores of silica is evident[Fig.1(B)(c)].HRTEM images of calcined KGM-1[Fig.1(B)(b),(d),(e)]indicate the presence of well-defined,uniform,and ordered nanometer-scale pores.Similar observation was noticed for porous MCM-41 having well-ordered hexagonal array.41,42Calcination of KGM-1destroys a portion of PANI chains present within the silica nanochannels and improves the contrast.As a result, the nanoscale architecture in the silica pores is visible.It is clear that the KGM-1has hexagonal-shaped ordered nano-pores[Fig.1(B)(e)].The elemental composition of KGM-1is as follows:72.11At%of C,1.29At%of N,10.27At%of Si,and4.34At%of S.KGM-1has a Si:N ratio of7.96,a value much higher than that of TMSPA($2.0),documenting the presence of three-dimensional silica network in KGM-1 (Scheme1).
FTIR spectrum of pristine KGM-1(a),neutralized KGM-1(b), and calcined KGM-1(c)are presented in Figure2.FTIR spec-tra of KGM-1,its neutralized form,and calcined form show the typical vibration bands of the silica framework.The band around1085cmà1and the shoulder around1220cmà1are assigned to the asymmetric Si-O-Si stretching modes.43The intensity of Si-O-Si peak increased on calcination of KGM-1 at540 C,indicating the presence of silica in KGM-1.FTIR spectrum of KGM-1displays bands around1590and1500 cmà1and are assigned to C??N and C??C stretching of qui-noid and benzenoid units in PANI chains.44The relative peak intensities ratio between C??C($1550cmà1)and C??N ($1590cmà1)stretching bands is the index of dopability for PANI chains in KGM-1.The ratios of intensities of C??C stretching to C??N stretching band(I C??C/I C??N)are compared between pristine KGM-1and neutralized KGM-1.The I C??C/ I C??N ratio of KGM-1(neutralized)is much higher($1.6) compared with pristine KGM-1($1.15)than as-prepared KGM-1.The higher ratio of I C??C/I C??N for KGM-1(neutralized) indicates that PANI chains are dedoped on deprotonation by treatment with ammonia.45These observations goes well with the UV-vis results discussed later.As can be observed, after heating KGM-1in vacuum at540 C,the peaks account-ing for PANI chain greatly diminished in intensity or were nearly absent.Calcination of KGM-1resulted in the decompo-sition of PANI chains in KGM-1.46Calcination of KGM-1at 540 C makes complete decomposition of PANI chains,leav-ing porous silica as white powder.These results offer strong evidences for the combined presence of silica and PANI units in KGM-1.
Further,the presence of PANI in KGM-1is evident from TGA results.Figure3shows the thermograms of pristine KGM-1 (curve a),neutralized KGM-1(curve b),and calcined KGM-1 (curve c).TGA of pristine KGM-1shows a three-step weight changes(Fig.3,curve a).A very small weight loss($2%)was witnessed below100 C because of evaporation of water, which indicated the purity of KGM-1($98%).The second weight loss($20%)between270and410 C is attributed to
JOURNAL OF POLYMER SCIENCE:PART A:POLYMER CHEMISTRY DOI10.1002/POLA
the removal of b-NSA(dopant)from the PANI chains in KGM-1and low-molecular-weight PANI that could be generated by partial fragmentation of PANI chains in KGM-1at these tem-perature ranges.47The third stage weight change($35%) that begins around470 C and extends upto610 C corre-sponds to the decomposition of PANI chains in KGM-1.How-ever,neutralized KGM-1shows only a two-step weight loss. The sharp and prominent weight loss($70%)between220 and280 C is possibly attributed to the decomposition of low-molecular-weight PANI.48The absence of dopant weight loss and the increased amount of weight loss($70%)at low temperature ranges(220–280 C)indicate that deprotonated PANI chains in KGM-1are susceptible for degradation to result in low-molecular-weight fragments.The weight loss ($5%)between410and540 C informs the decomposition of residual PANI units in KGM-1(neutralized).Thermograms of calcined KGM-1(curve c)do not show any significant weight changes upto600 C.This reveals that KGM-1is con-verted to silica on calcination.
UV-vis spectra(Fig.4)of pristine KGM-1(curve a)and KGM-1 (neutralized)(b)are presented.Both spectra reveal the characteristic electronic bands of PANI in KGM-1.The
FIGURE1(A)Low-angle XRD pattern of KGM-1. Inset:Wide-angle pattern of KGM-1.(B)HRTEM image of KGM-1(a)and KGM-1(calcined)(b). High-magnification image of KGM-1(c)and KGM-1(calcined)(d).(e)Magnified image of the boxed area in
(d).
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spectrum of KGM-1(curve a)exhibited three bands around 320,520,and 600nm.The bands around 320and 520nm are assigned to p -p *transition and polaron-p *transi-tions,respectively.The band around 600nm corresponds to transition from the p state of undoped PANI chains.For the pristine KGM-1,the peak intensity of polaronic band (520nm)is much higher than the p state of undoped PANI chains.In the UV-vis spectrum of KGM-1(neutral-ized)(curve b),the band corresponding to polaron transi-tion ($520nm)diminished in intensity and the band cor-responding to undoped state ($600nm)increased in intensity.The changes in electronic bands between KGM-1and KGM-1(neutralized)clearly revealed that PANI chains in KGM-1are dedoped by neutralization.Calcined KGM-1does not show any spectral bands in the region 300–800nm
(spectrum not shown).This could be due to the loss of PANI chains in KGM-1on calcination.
The N 2adsorption–desorption studies of KGM-1,KGM-1(neutralized),and KGM-1(calcined)(Fig.5)reveal the typi-cal IV-type adsorption as defined by IUPAC.49KGM-1is nano-porous as evident from N 2adsorption–desorption measure-ments.KGM-1has a pore diameter of 2.4nm,a pore volume of 1.6?10à2cm 3/g,and a surface area of 40m 2/g.The pores are narrow,indicating that PANI is formed within silica pores as determined from Brunauer–Emmett–Teller isotherm and Barrett–Joyner–Halenda method.50,51The wider wall thickness (6.11nm)arises from the hydrolysis of silicate through three-dimensional cross-linking processes.The much higher wall thickness for KGM-1(74?)corroborates with the higher Si:N ratio.KGM-1has a definite proportion of S,with S:N ratio as 0.36.These results corroborates with the formation of three-dimensional silica network and doped state of PANI chains in KGM-1.The conductivity (8?10à4S/cm)of KGM-1is lower compared with conventional PANI.KGM-1exhibits H1-type narrow hysterisis loop,with a change in adsorption around P /P 0?0.20.The surface area and pore size of KGM-1are 49m 2/g and 2.7nm,respec-tively.Calcined KGM-1exhibits the H1-type narrow hysterisis loop,characteristic of materials having uniform pores with open cylindrical geometry.52The surface area of calcined KGM-1is 640m 2/g ($16times higher than KGM-1),and the average pore size is 3.54nm.Thus,N 2adsorption features also inform that PANI is present as an uniform layer within the pores of silica and because of that the silica pores are not completely blocked.If PANI is formed heterogeneously in the cylindrical pores of silica,the channels in silica would be plugged.A broad hysterisis in N 2adsorption isotherm could be resulted.On neutralization of KGM-1,the b -NSA ions that exist as dopants to PANI units are removed.As a result,a slight increase in the surface area was noticed.However,on calcination,organic portions (PANI)of the hybrid
materials
FIGURE 2FTIR spectrum of KGM-1(a),KGM-1(neutralized)(b),and KGM-1(calcined)(c).[Color figure can be viewed in the online issue,which is available at
https://www.doczj.com/doc/692847681.html,.]
FIGURE 3Thermograms of KGM-1(a),KGM-1(neutralized)(b),and KGM-1(calcined)(c).[Color figure can be viewed in the online issue,which is available at
https://www.doczj.com/doc/692847681.html,.]
FIGURE 4Electronic properties of KGM-1(a)and KGM-1(neu-tralized)(b).[Color figure can be viewed in the online issue,which is available at https://www.doczj.com/doc/692847681.html,.]
JOURNAL OF POLYMER SCIENCE:PART A:POLYMER CHEMISTRY DOI 10.1002/POLA
are removed,and a significant increase in surface area and pore size are observed.
Nanolayer KGM-1films of defined thickness were deposited by cyclic voltammetry.CVs recorded for the layer by layer deposition of KGM-1onto ITO are presented [Fig.6(A)].The progressive increase in the peak current values at the an-odic peaks around $0.50V and $0.60V,and reduction waves at $0.40V and $0.60V,informs the continuous build up of electroactive KGM-1.The redox peaks ($0.50and 0.60V)are assigned to the reversible conversion of leuco emeraldine (LE)to emeraldine (E)and E to pernigra-niline (P)type structure of the PANI units in KGM-1.The conversion of LE to E form of PANI occurs at a much lower potential ($0.1–0.3V).However,for KGM-1,interconversion of LE to E occurs at a $0.50V.The difference is attributed to the presence of insulating silica frame work.CVs [Fig.6(A)]recorded at various potential cycles during the deposition of KGM-1film inform that the thickness of the film was around $15nm at the end of second potential cycle and gradually increased by 80–100nm per potential cycle.The film of KGM-1exhibits stable and reversible redox characteristics [Fig.6(A),inset].
UV-vis spectroelectrochemical studies were performed for the film of KGM-1[Fig.6(B),inset]to investigate the electro-chromic changes of KGM-1.KGM-1film showed multiple color changes with applied potential (Table 1).Spectroelec-trochemical studies reveal that KGM-1exhibits a reversible electrochromic behavior with a change in color from yellow at à0.10V to yellow green at t0.50V and to dark green at t0.70V.To the best of our knowledge,this is the first report
on the electrochromic behavior of silica–PANI hybrid nano-structured material.
Electrochemical characteristics [Fig.6(A)]and N 2adsorp-tion–desorption measurement (Fig.5)revealed that KGM-1could exhibit reversible redox transitions that correspond to PANI and nanoporous,respectively.These features are effec-tively used for the electro-driven loading/release of DNA or ATP [Fig.6(C)].We have recorded the fluorescence images of chromophore-tagged DNA 53by applying potentials at the film of KGM-1for probing the loading/release of DNA mole-cules (Supporting information S1).At t0.80V,the major proportion of DNA molecules are loaded within the nano-pores of KGM-1,and the DNA molecules are rarely seen at the film surface.At t0.80V,PANI chains linked to silica net-work exist in E state,and DNA molecules are pulled within the nanochannel of silica as the dopant.And,at the far nega-tive potential (à0.4V),majority of DNA molecules are found at the surface of ITO/KGM-1electrode because of the release of DNA from the nanochannels (Supporting information S1).Electrochemically stimulated release of ATP has been moni-tored at PANI-ATP films.54Results indicated that ATP mole-cules loaded within films of CPs leached out to the environ-ment (medium).However,the rigid silica frame work and nanoporous nature of KGM-1provide stable environment for the biomolecules.KGM-1exhibits a stable and reproducible electrochemical response for the loading/release of DNA.In the absence of KGM-1film,the DNA molecules are found throughout the surface of ITO electrode irrespective of the applied potentials.Further,we hypothecate that the DNA-loaded KGM-1could be conjugated with other
targeting
FIGURE 5(A)Nitrogen adsorption–desorption isotherm for KGM-1(a),KGM-1(neutralized)(b),and KGM-1(calcined)(c).(B)Pore size distribution of KGM-1(a),KGM-1(neutralized)(b),and KGM-1(calcined)(c).
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FIGURE 6(A)Cyclic voltammogram recorded during the formation of KGM-1(TMSPA ?50m M ,NSA ?1M ;Scan rate:100mV/s).Inset:Cyclic voltammogram of film of KGM-1(NSA ?1M ;Scan rate:100mV/s).(B)UV-vis spectra of film of KGM-1.Inset:In situ spectroelectrochemical analysis of KGM-1.UV-vis spectra were collected at different applied potentials:(a),à0.1V;(b),0.1V;(c),0.3V;(d),0.5V;and (e),0.7V.(C)Scheme showing the electrodriven loading/release of DNA.[Color figure can be viewed in the online issue,which is available at https://www.doczj.com/doc/692847681.html,.]
TABLE 1Spectroelectrochemical Properties of KGM-1Different Forms of KGM-1Color Adsorption Band at Different Potentials
à0.1V t0.60V t1.0V
LE form Yellow 410nm 530nm Beyond 800nm E form Bluish green 440nm 590nm 780nm P form
Dark green
370nm
–
780nm
JOURNAL OF POLYMER SCIENCE:PART A:POLYMER CHEMISTRY DOI 10.1002/POLA
components such as specific ligands or antibodies for devel-oping a smart drug delivery system.
CONCLUSIONS
This is the first report on the simple,one-pot preparation of silica-PANI nanohybrid having versatile properties.KGM-1is nanoporous(pore diameter of2.4nm),electroactive(stable reversible redox characteristics),and electrochromic(yellow atà0.10V,yellow green att0.50V,and dark green at t0.70V).KGM-1possesses the essential features for wider applications covering DNA-biochips,biochemical sensing,bio-fuel cells and catalyst,and so forth.
This work was supported by Priority Research Centers Pro-gram through the National Research Foundation of Korea (NRF)funded by the Ministry of Education,Science,and Technology(2009-0093819)and the Ministry of Science and Technology(MOST)through its National Nuclear Technology Program(2007-2000794).The authors acknowledge the Korea Basic Science Institute(Deajon)and Kyungpook National Uni-versity Center for Scientific Instrumentation. REFERENCES AND NOTES
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JOURNAL OF POLYMER SCIENCE:PART A:POLYMER CHEMISTRY DOI10.1002/POLA