1-s2.0-S0925400515002981-main

Sensors and Actuators B 214 (2015) 8291

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

jo u r nal homep age: www.elsev ier .com/ locate /snb

Development of selective and sensitive bicarbonabased on wet-chemically prepared CuO-ZnO nan

Mohammed M. Rahman , Abdullah M. AsiriChemistry Dep laziz UJeddah 21589,

a r t i c l

Article history:Received 2 AuReceived in reAccepted 26 FAvailable onlin

Keywords:CuO doped ZnBicarbonate seWet-chemicalSensitivityIV techniqueReal water samples

xide nts in, ene

microponse, reliacal petratio2 anpose

detection of bicarbonate in various environmental real water samples with acceptable and reasonableresults.

2015 Elsevier B.V. All rights reserved.

1. Introdu

A chemnal proportanalytes. Thprinciples enhanced ials whetherin chemicaattracted apotential apductor matnano-materrevealed a sber of irregoxide atomTransition m

Corresponand ChemistryP.O. Box 80203fax: +966 0269

E-mail add(M.M. Rahman

http://dx.doi.o0925-4005/ ction

ical sensor is a sensor that produces an electric sig-ional to the concentration of chemical or biochemicalese chemi-sensors use chemical as well as physical

in their operation. The sensor signal is signicantlyn the presence of nanomaterials or functional materi-

doped or undoped fabricated onto the sensor surfacesl or biological systems. Doped nanomaterials have

wide interest due to their unique properties andplication in chemi-sensor fabrication [1,2]. Semicon-erial has been also recognized as a promising hostial for transition metal oxides at room conditions. It istable morphological structure and composed of a num-ular phases with geometrically coordinated metals ands, piled alternately along the three dimensional axes [3].etals codoped in semiconductor nano-materials have

ding author at: Center of Excellence for Advanced Materials Research Department, Faculty of Science, King Abdulaziz University,, Jeddah 21589, Saudi Arabia. Tel.: +966 59 6421830;52292.resses: [email protected], [email protected]).

concerned insightful research effort for its exceptional and out-standing optical, structural, electrical, photocatalytic properties,and versatile applications [4]. Last decade, an extensive devel-opment has been executed on the research leading of transitionmetal oxides incorporated ZnO materials actuated by both fun-damental sciences and prospective advanced technologies [57].The doped semiconductor nanostructures exhibit promising usesas eld consequence transistors [8], UV photo-detectors [9,10],gas sensors [11], eld emission electron sources [12], nanomate-rials [13,14], nanoscale power generators [15], and many otherfunctional devices [16]. Transition metal doped nanostructure isalso an effective method to regulate the energy level in surfacestates of ZnO, which can further progress by the changes in dopingconcentrations of host-guest materials. Chemical sensing expedi-tion has been utilized with the metal oxide nanostructures for thedetection of various chemicals such as ethanol, phenyl hydrazine,methanol, hydrazine, chloroform, dichloromethane, acetone, andethanol which are not environmental friendly. The sensing mech-anism with doped metal oxides thin lms utilized mainly theproperties of porous lm formed by the physi-sorption and chemi-sorptions methods. The chemical detection is based on the currentchanges of the fabrication thin lms caused by the chemical com-ponents of the reacting system in aqueous medium [1720]. Themain effort is focused on detecting the minimum quantity aqueous

rg/10.1016/j.snb.2015.02.1132015 Elsevier B.V. All rights reserved.artment and Center of Excellence for Advanced Materials Research (CEAMR), King AbduSaudi Arabia

e i n f o

gust 2014vised form 23 February 2015ebruary 2015e 9 March 2015

O nanorodsnsors

method

a b s t r a c t

We have prepared calcined copper ochemical method using reducing ageby UV/vis, FT-IR, X-ray photoelectronand eld-emission scanning electronto result in a sensor that has a fast reshigh sensitivity, lower-detection limitselective, and enhanced electrochemilinear (r2 = 0.99) over the large concenare calculated as 1.667 A cm2Mtively. Finally, the efciency of the prote chemical sensororods

niversity, P.O. Box 80203,

doped zinc oxide nanorods (CuO-ZnO NRs) by a facile wet- alkaline medium. The doped NRs were totally characterizedrgy-dispersive X-ray spectroscopy, X-ray powder diffraction,scopy. The NRs were deposited on at silver electrode (AgE)

to selective bicarbonate in buffer system. Features includingbility, reproducibility, ease of integration, long-term stability,rformances were investigated in detail. The calibration plot isn range (1.0 nM to 1.0 mM). The sensitivity and detection limitd 0.89 0.02 nM (at a signal-to-noise-ratio, SNR of 3) respec-d chemi-sensors can be applied and effectively utilized for the

M.M. Rahman, A.M. Asiri / Sensors and Actuators B 214 (2015) 8291 83

chemical necessary for the fabrication doped sensors for electro-chemical investigation. Nanomaterials offer many opportunities oftuning the chemical sensing properties.

In physiological systems, carbon dioxide is obviously linked withbicarbonatelogical procthe level ofthe discoveby bicarbonchemi-senssenger cyclthe evidenctional, subscyclases, fuchemi-sensphysiologicproducts ofing to Eq. (1water repre

Glucose (or

CO2 + H2O In unice

away, but onmethods focombines wciates to libBicarbonateminutes, buinstantaneodrases [22,2in intercelluinterplay beproton trantrial vertebin two wayand the brebon dioxideexquisitely tration of ban appropraddition toby bicarbontion, blood formation),sor. For manascribed tobonate wercellular pH.modulates the enzymeadenosine HCO3/carbger signalliand pH arvariety of blution, depBicarbonateHCO3, carmonophospdioxide/pHThe future chemi-sensBicarbonatecyclases an

cyclic nucleotide signalling is an evolutionarily conserved mecha-nism for HCO3/carbon-dioxide/pH chemi-sensing.

In the present study, it is employed the wet-chemical tech-nique to prepare CuO doped ZnO semiconductor nanomaterials

earlys motentidopeted tomatical y elructuuid/soperologg prohemde surbon

our withque i

erim

ateri

c chl acer cSigmmax

usinny (Rs tome/wweasuttp:/adiaze wnalyV. Th8 Toere r

https of

m PAed wvoltppli

o meon s

frowwwnO N/ww

nthe

-ZnOtion al re

diss (HCO3) via carbonic anhydrases, and numerous bio-esses are dependent upon a mechanism for sensing

HCO3 or carbon dioxide, and/or pH. In human body,ry that soluble adenylyl cyclase is directly regulatedate (HCO3) provided a link between HCO3/CO2/pHing and signalling through the widely used second mes-ic adenosine monophosphate. This review summarizese that bicarbonate-regulated adenylyl cyclase, and addi-equently identied bicarbonate-regulate nucleotidylnction as evolutionarily conserved HCO3/pH/CO2ors in a wide variety of physiological systems [21]. Inal system, carbon dioxide and water are the major end

energy producing pathways in living organisms accord-). As such, in non-photosynthetic organisms, CO2 andsent the most fundamental catabolites.

other energy sources) + O2 CO2 + H2O (1) H2CO3 HCO3 + H+ (2)llular organisms, carbon dioxide gas can simply diffusece multi-cellular organisms evolved, they had to deviser safely dealing with carbon dioxide. In solution, CO2ith water to form carbonic acid (H2CO3), which disso-erate a proton and a bicarbonate ion (HCO3) (Eq. (2))., carbon dioxide and pH equilibrate on their own withint in biological systems, equilibrium is reached nearlyusly due to the ubiquitous presence of carbonic anhy-3]. This equilibrium is used to buffer pH inside cells andlar uids; for example, intracellular pH is regulated viatween carbon dioxide diffusion, and bicarbonate andsporters and/or exchangers. In mammals, and terres-rates in general, this equilibrium is tightly controlleds; the kidneys regulate the bicarbonate concentrationathing frequency determines the concentration of car-. Each of these processes requires a sensor i.e., ansensitive and rapid way to measure the precise concen-icarbonate and/or carbon-dioxide and/or pH and elicitiate response. Many other physiological processes, in

diuresis and breathing rate regulation, are modulatedate and/or carbon dioxide and/or pH (i.e., sperm activa-ow, aqueous humour in the eye and cerebrospinal uid

and they also require a HCO3/carbon-dioxide/pH sen-y years, the effects of carbon dioxide and pH had been

undened chemo-receptors, and the effects of bicar-e traditionally thought to be mediated by changes in

In 2000, researcher demonstrated that HCO3 directlythe activity of soluble adenylyl cyclase, a novel form of

generating the ubiquitous second messenger, cyclicmonophosphate [2426], revealing that physiologicalon dioxide/pH could be sensed via second messen-ng. In physiological systems, HCO3, carbon dioxide,e intimately linked via carbonic anhydrases and aiological processes, in mammals and throughout evo-end upon a HCO3/carbon-dioxide/pH chemosensor.-regulated adenylyl cyclase, which links intracellularbon-dioxide, and/or pH levels with cyclic adenosinehate signal transduction, serves as the HCO3/carbon-

chemi-sensor in at least a subset of these processes.will reveal whether other HCO3/carbon-dioxide/pHing functions are also mediated by adenylyl cyclase.

regulation is observed in other mammalian nucleotidyld in adenylyl cyclases across evolution implying that

with ntinuouand poon undedicator nanthe opof mannanostthe liqical prmorphsensincient celectroof bicabest ofbonatetechni

2. Exp

2.1. M

Zincarbitoall othfrom The cuted GermaZnO Ntropho(http:/was meter (hX-ray rspot sixed a200.0 eless 10NRs wJapan;patternter froequipperator were aused tradiatichasedhttp://CuO-Z(http:/

2.2. Sy

CuOcentrachemicslowly controlled rod-shape structure, which revealed a con-rphological advancement in nanostructure materialsal applications. With most of the existing works focusedd ZnO, there have been more and more attentiono explore the doped counterparts. For semiconduc-terials, doping is an inuential application to conformand electrical properties, expediting the developmentectronic and opto-electronic devices. Semiconductorres CuO-ZnO NRs allow very sensitive transduction ofurface interactions into a change in the electrochem-ties. The possibility is to form a variety of structuralies suggests various prospects of tuning the chemicalperties. CuO-ZnO NR is fabricated by a simple and ef-ical sensors consisting on a side-polished at silverrface; and measured the chemical sensing performanceate in phosphate buffer system at room conditions. Toknowledge, this is the rst report for detection of bicar-

prepared CuO-ZnO NRs using simple and reliable IVn short response time.

ental

als and methods

loride, copper chloride, sodium bicarbonate, butyletate, ethyl acetate, ammonia solution (25.0%), andhemicals were in analytical grade and purchaseda-Aldrich Company (http://www.sigmaaldrich.com).(366.0 nm) of calcined CuO-ZnO NRs was exe-

g UV/visible spectroscopy Lamda-950, Perkin Elmer,http://www.perkinelmer.com). FT-IR spectra of CuO-were performed on a spectrum-100 FT-IR spec-ter in the mid-IR range purchased from Brukerw.bruker.com). The XPS measurement of CuO-ZnO NRsred on a Thermo Scientic K-Alpha KA1066 spectrom-/www.thermoscientic.com). A monochromatic Al K1tion source was used as excitation sources, where beam-as kept in 300.0 m. The spectra were recorded in thezer transmission mode, where pass energy was kept ate scanning of the spectra was performed at pressuresrr. Morphology, size, and structure of calcined CuO-ZnOecorded on FESEM instrument from JEOL (JSM-7600F,://www.jeol.com/). The powder X-ray diffraction (XRD)

CuO-ZnO NRs were recorded by X-ray diffractome-Nalytical diffractometer (http://www.panalytical.com)ith Cu K1 radiation ( = 1.5406 nm) using a gen-

age of 45.0 kV and a generator current of 40.0 mAed for the determination. Raman spectrometer wasasure the Raman shift of calcined CuO-ZnO NRs usingource (Ar+ laser line, ; 513.4 nm), which was pur-m Perkin Elmer (Raman station 400, Perkin Elmer,.perkinelmer.com). IV technique is measured withRs fabricated at AgE by using Kethley-Electrometerw.keithley.com) from USA.

sis and growth mechanism of CuO-ZnO NRs

NRs have been synthesized by adding uni-molar con-of copper chloride and zinc chloride precursor intoactors for 6 h. Copper chloride and zinc chloride wereolved separately into the de-ionized water to make

84 M.M. Rahman, A.M. Asiri / Sensors and Actuators B 214 (2015) 8291

Sch ds. Ca

0.1 M concesolutions wsolution pHtion (25.0% conical uxmaintainedand NH4OHtion for co-pNH4OH wasCuCl2 solutimechanismchemical reZnO crystalobtaining th

NH4OH(aq) ZnCl2(s) + 2

Zn(OH)2(aq)

CuCl2(aq) + 4+ 2Cl(aq

Zn(OH)2(aq)

The reacline mediuaddition ofwas stirredsolution pHto set into othe solutionreaction is the solutionwater and total synthevalue of ththe concencritical valuthere is higtion of ZnOthe lower aHowever, aof solutionogy developdoped ZnOapproximat[29,30]. In dZnO nucleure-aggregat

nown-aggrms

in Scalcine, 60terizral, eliab

brica

M ph 0.2 ize wnO N

a cot 70.0ctrocg eleodiurentt of houtariouo 1.0. conue ofused.

ults

alys

calcoleclearlployuO-eme 1. Growth mechanism of wet-chemically prepared calcined CuO-ZnO nanoro

ntration at room temperature. Then these equi-molarere mixed gently and stirred until mix properly. The

was slowly adjusted drop wise by diluted alkaline solu-ammonium hydroxide). Then the mixture was put into

(reactor cell) to put in the oven for linger time and the temperature. The starting materials of ZnCl2, CuCl2,

(reducing agent) were used without further purica-recipitation method to CuO into ZnO nanocrystals. The

added drop wise into the vigorously stirred ZnCl2 andons mixture to produce a white precipitate. The growth

of the nanomaterials could be explained on the basis ofactions and nucleation, as well as the development ofs. The apparent reaction mechanisms are proposed fore doped metal oxides, which are presented below:

NH4+(aq) + OH(aq) (3)OH(aq) Zn(OH)2(aq) + 2Cl(aq) (4)

+ 2NH4OH(aq) ZnO21(aq) + 2NH4+(aq) + 2H2O (5)

NH4OH(aq) + ZnO21(aq) CuO-ZnO(s) + 4NH4+(aq)) + 2H2O (6)

+ CuO(aq) CuO-ZnO(s) + H2O (7)tant precursors of CuCl2 and ZnCl2 are soluble in alka-m (NH4OH reagent) according to Eqs. (3)(5). After

NH4OH into the mixture of metal oxides solution, it gently for few minutes at room temperature. Then the

is adjusted (10.0) using NH4OH and put into reactorsven at 95.0 C for 6 h, where the active temperature of

mixture is approximately in the range of 90.0 C. Theprogressed slowly according to Eqs. (6) and (7). Then

was washed thoroughly with ethanol, acetone, andthen kept for drying at room temperature. During thesis process, NH4OH acts a pH buffer to regulate the pHe solution and slow supply of OH-ions [27,28]. Whentration of the Zn2+ and OH ions is reached over thee, the precipitation of ZnO nuclei becomes started. As

well-kand reand fosentedwere cmolyncharacstructuusing r

2.3. Fa

0.1 mixingde-ionCuO-Z(EA) asoven aAn eleworkinpared sat diffeAmounthrougwith v1.0 M trent vsthe valeter is system

3. Res

3.1. An

Theand mtions care emcined Ch concentration of Cu2+ ion in the solution, the nuclea- crystals become formed co-precipitation, owing toctivation energy barrier of heterogeneous nucleation.s the concentration of Cu2+ presences in the mixture, some larger ZnO crystals with a rod-like morphol-ed among the nanostructures, which composed of CuO

nanomaterials. The shape of CuO doped ZnO rods isely consistent with the growth habit of ZnO crystalsoped nanorods growth mechanism, initially CuO ands growth takes place by self-aggregation, which thenes and produced CuO-ZnO nanocrystal according to the

observed vi(MOM) sfrequenciesdoped nano

The optiusing UV-vtion spectrFig. 1B. It rerange betwthe formati[33,34]. Banlcinations temperature set in mufe-furnace was 450 C.

Ostwald ripening method. Nano-material crystallizesregates with each other through Vander-Waals forcesdoped CuO-ZnO nanorods morphology, which is pre-heme 1. Finally, the as-grown CuO doped ZnO nanorodsed at 450.0 C for 3 h in the furnace (Barnstead Ther-00 Furnace, USA). The calcined products were totallyed in detail in terms of their morphological, elemental,optical properties, and applied for bicarbonate sensingle IV methods.

tion and detection technique of bicarbonate

osphate buffer solution (PBS) at pH 7.0 is prepared byM Na2HPO4 and 0.2 M NaH2PO4 solution in 100.0 mLater. Flat silver electrode is fabricated with calcinedRs, where butyl carbitol acetate (BCA) and ethyl acetatenducting coating agent. Then it is transferred into theC for 4 h until the lm is completely uniform and dried.hemical cell is mounted with NRs coated at AgE as actrode and Pd wire is used a counter electrode. As pre-m bicarbonate (NaHCO3, stock solution 1.0 M) is diluted

concentrations in buffer solution and used as a target.0.1 M PBS was kept constant in the beaker as 10.0 mL

the chemical investigation. Analyte solution is prepareds concentrations of aqueous sodium bicarbonate from

nM. The sensitivity is calculated from the slope of cur-centration (I vs. C) from the calibration plot divided by

active surface area of sensors/AgE electrodes. Electrom- as a voltage sources for IV technique in two electrode

and discussion

is of optical and structural properties

ined CuO-ZnO NRs is also characterized from the atomicular vibrations. To anticipate the actuated recogni-y, FT-IR spectra only in the region of 4004000 cm1

ed. Fig. 1A represents the FT-IR spectrum of the cal-ZnO nanomaterials. It exhibits a band at 745 cm1. This

bration band may be assigned as metaloxygenmetaltretching vibration. The observed vibration bands at low

regions suggest the formation of metal-oxide bond in-materials [31,32].cal absorption spectra of CuO-ZnO NRs are executed byis spectrophotometer in the visible range. The absorp-um of calcined doped NRs solution is presented inpresents the absorption maxima at 366.0 nm in visibleeen 200.0 and 800.0 nm wavelengths, which indicatedon of CuO-ZnO NRs formation by wet-chemical routed gap energy is calculated on the basis of the maximum


-ray d

absorption according t

Ebg =1240

where Ebg(366.0 nm)

The CuOwurtzite strat 450.0 C ition of nanothe calcinedtion peaks having baserepresentedfor calcined(1 1 2), ((1 1 4), anare a = 4.66radiation: Cnicant amthe reectiophase (zincThe phaseswith indice(0 0 2), (1 0 hexagonal (point groupcate that thnano-materbe indexed responding

opeFig. 1. (A) FT-IR spectroscopy, (B) UV/visible spectroscopy, and (C) powder X

band of CuO-ZnO NRs and obtained to be 3.3879 eV, oxide d

o following equation:

(eV) (8)

is the band-gap energy and max is the wavelengthof the NRs.-ZnO NRs samples were analyzed and exhibited asucture with hexagonal shapes. The sample was calcinedn mufe furnace to improve the crystallinity or forma-crystalline phases. Fig. 1C shows typical crystallinity of

CuO-ZnO nanorods and their aggregation. The reec-were observed to match with CuO phase (Tenorite)-centred monoclinic [JCPDS # 073-6234]. The phases

the major characteristic peaks (symbol, *) with indices crystalline CuO at 2 values of (1 1 0), (1 1 1), (1 1 1),2 0 2), (0 2 0), (2 0 2), (1 1 3), (1 1 3), (2 2 1), (2 2 2),d (3 1 2) degrees. The monoclinic lattice parameters2, b = 3.417, c = 5.118, = 99.48, point group: C2/c, andu K1 ( = 1.5406). These indicate that there is sig-ount of crystalline CuO present in nano-materials. Alln peaks in this pattern were found to match with ZnOite) having hexagonal geometry [JCPDF # 071-6424].

showed the major characteristic peaks (symbol, #)s for calcined crystalline ZnO at 2 values of (1 0 0),1), (1 0 2), (1 0 3), (2 0 0), (2 0 1), and (2 0 3) indices. Theunit cell) lattice parameters are a = 3.2494, b = 5.2038,: P63mc, and radiation: Cu K1 ( = 1.5406). These indi-ere is signicant amount of crystalline ZnO present inials. The diffraction patterns of calcined samples canto the hexagonal structure of ZnO. The other peaks cor-

to CuO related secondary phase was found in copper

of CuO intoThe crys

the well-kn

D = 0.9 cos q

where is half maximaverage dia

3.2. XPS an

Quantitatroscopy, Xanalysis of dmethod thapresent witating on Cudetermininaway from XPS was alNRs and theHere, XPS msemiconduCuO and Znsented in Fiin Fig. 2B. Tindicated to[37]. In Fig.binding eniffraction pattern of calcined CuO-ZnO nanorods.

d sample, which may be attributed to the incorporation

the ZnO lattice site [35,36].talline size was also calculated and conrmed by usingown Scherrer formula (9):

(9)

the wavelength of X-ray radiation, is the full width atum (FWHM) of the peaks at the diffracting angle . Themeter of CuO-ZnO NRs is close to 42.5 nm.

alysis

tive spectroscopic method (X-ray photoelectron spec-PS) is studied in details to determine the elementaloped CuO-ZnO NRs. XPS is a quantitative spectroscopict determines the chemical-states of the elements thathin doped materials. XPS spectra are acquired by irradi-O-ZnO NRs with a beam of X-rays, while simultaneouslyg the kinetic energy and number of electrons that get-the top one to ten nm of the material being analyzed.so used to resolve the chemical state of the CuO-ZnOir depth of binding energy of each element with others.easurements were measured for doped CuO-ZnO NRs

ctor nanomaterials to investigate the chemical states ofO. The XPS spectra of Cu2p, Zn2p, and O1s are pre-g. 2A. The O1s spectrum shows a main peak at 528.5 eVhe peak at 528.5 eV is assigned to lattice oxygen may be

oxygen (i.e., O2) presence in the doped CuO-ZnO NRs 2C, the spin orbit peaks of the Cu2p(3/2) and Cu2p(1/2)ergy for all the samples appeared at around 934.3 eV


Fig. 2. XPS of (A) doped CuO-ZnO NRs, (B) O1s level, (C) Cu2p level, and (D) Zn2p level acquired with Mg K1 radiation.

and 956.72reference dZn2p(1/2) anat around 1agreement tional analyof CuO andchemical prtwo materinoticeably.

orph

EM D. sionated ds (

thaructusses

Fig. 3. (A eV respectively, which is in good agreement with theata for CuO [38]. In Fig. 2D, the spin orbit peaks of thed Zn2p(3/2) binding energy for all the samples appeared023.8 eV and 1049.2 eV respectively, which is in goodwith the reference data for ZnO [39,40]. XPS composi-ses evidenced the co-existence of the two single-phase

ZnO materials. Therefore, it is concluded that the wet-epared CuO-ZnO materials have NRs phase containedals. Also, this conclusion is reliable with the XRD data

3.3. M

FESFig. 3Adimencalculananoroimagesnanostand poD) Low to high magnied FESEM images of calcined CuO-ZnO NRs. Probe current: 7.0; beological analysis

images of CuO-ZnO NRs structures are presented inIt exhibits the images of the rod-shapes with nano-l sizes of CuO-ZnO NRs. The dimension of NR isin the range of 35.060.0 nm, which composed of43.0 10.0 nm). It is clearly exposed from the FESEM

t the facile wet-chemically synthesized CuO-ZnO isres in rod-shape, which is grown in very high-density

sing almost uniform nanorods. When the size of dopedam: 15.0 KeV; mode: SEM; magnication: 14,000 to 170,000.


Fig. 4. Ele

material deincreases smaking re-dsized CuO-Zis an agreem

3.4. Elemen

The X-rathese CuO-(Zn), and onanostructudisplayed thCu, Zn, and which is preimpurity hananoparticl

4. Chemica

4.1. Fabrica

The prosAgE as chemexecuted foment of dopsensors is able. The Cuin air, non-ity, simpliccharacterist

2. Scinder

isms oterials

nalensided. Ti-see fanduentet intostabl

chelm asponsmental analysis by XEDS of calcined doped CuO-ZnO nanorods.

Schemecoating bmechannanoma

of ratioNRs coadsorbof chemlyte. Thwith cois preswas puit dry, bonatethin-cal rescreases into naonometer-sized scale, the surface areaignicantly, this improves the energy of the system,istribution of Zn and Cu ions possible. The nanometer-nO NRs could have tightly packed into the lattice, whichent with the publish reports [4143].

tal analysis

y electron dispersive spectroscopy (XEDS) analysis ofZnO NRs is indicated the presence of copper (Cu), zincxygen (O) composition in the pure calcined dopedre material, which is presented in Fig. 4A. It is clearlyat the calcined prepared nanomaterials contained onlyO elements with the 2.18, 73.52, 24.3 wt% respectively,sented in Fig. 4B (inset). No other peak related with anys been detected in the XEDS, which conrms that thees are composed only with Cu, Zn, and O.

l sensor application

tion of bicarbonate sensor using CuO-ZnO NRs

pective application of CuO-ZnO NRs assembled onto atical sensors (especially bicarbonate analyte) has beenr measuring and detecting target chemical. Enhance-ing of this prepared CuO-ZnO NRs on AgE as chemicalin the initial stage and no other reports are avail-O-ZnO NRs sensors have advantages such as stabilitytoxicity, chemical inert-ness, electro-chemical activ-ity to assemble, ease in fabrication, and chemo-safeics. As in the case of bicarbonate sensors, the incident

reliable IVis presenteset for 1.0 s.applied potreaction mebicarbonatebicarbonateence of dopcurrent resproom condi

4.2. Detecti

Fig. 5A sand coatedNRs. With Npared to unis slightly ithe NRs moinjecting ofied electroof surface get bicarbowith CuO-Zthe variousis showed CuO-ZnO Nin room cocentration were increato higher phematic view of (A) and (B) uncoated and coated of AgE with NRs ands, (C) detection IV method (theoretical), and (D) proposed adsorptionf bicarbonate detection in the presence of semiconductor CuO-ZnO. Surface area of AgE: 0.0216 cm2; method: IV.

is that the current response in IV method of CuO-ZnOerably changes when aqueous bicarbonate analytes arehe calcined CuO-ZnO NRs were applied for fabricationnsor, where bicarbonate was measured as target ana-bricated-surface of CuO-ZnO NRs sensor was preparedcting binders (EC and BCA) on the at AgE surface, whichd in Scheme 2(A) and (B). The fabricated AgE electrode

the oven at low temperature (60.0 C) for 2.0 h to makee, and uniform the surface totally. IV signals of bicar-mical sensor are anticipated having CuO-ZnO NRs on

a function of current vs. potential. The resultant electri-es of target bicarbonate are investigated by simple and

technique using CuO-ZnO NRs fabricated AgE, whichd in Scheme 2C. The holding time of electrometer was

A signicant amplication in the current response withential is perceptibly conrmed. The simple and possiblechanism is generalized in Scheme 2D in the presence of

on CuO-ZnO NRs sensor surfaces by IV method. The is converted to water and carbon dioxide in the pres-ed nanomaterials, which improved and enhanced theonses against potential during the IV measurement attions.

on of bicarbonate by CuO-ZnO NRshows the current responses of uncoated (grey-dotted) (dark-dotted) AgE electrode surfaces with CuO-ZnORs fabricated surface, the current signal is reduced com-coated surface, which indicates the modied surfacenhibited with CuO-ZnO NRs. The current changes fordied lm before (dark-dotted) and after (blue-dotted)

50.0 L bicarbonate (1.0 nM) onto CuO-ZnO NRs mod-de, which is presented in Fig. 5B. This signicant changecurrent is investigated in every injection of the tar-nate onto the NRs/AgE by electrometer. IV responsesnO NRs modied sensor surface are investigated from

concentrations (1.0 nM to 0.1 M) of bicarbonate, whichin Fig. 5C. It shows the current changes of fabricatedRs/AgE lms as a function of analyte concentrationndition. It was also found that at low to high con-of target bicarbonate analyte, the current responsessed gradually. The potential current changes at lowerotential range (potential, +0.10 V to +1.5 V) based on


Fig. 5. IV resbicarbonate (1

various anasented in Fiwith the prto 0.1 M. Tof bicarbonFig. 5D. Thwhich is cloof this bicarR = 0.99; Fig0.89 0.02 abbreviatedneering thaback-grounthe noise m

IV charfunction ofcurrent rescurrent resincreasing chave also bcentration surface covhence the subonate condue to a larresulting inincrease inlm, whichponses of (A) bare AgE and NRs/AgE; (B) NRs/AgE (in the absence of bicarbonate) and NRs.0 nM to 0.1 M); and (D) calibration plot (at +0.5 V) of CuO-ZnO NRs/AgE. Magnied view

lyte concentration are observed, which is clearly pre-g. 5C. A large range of analyte concentration is executedobable analytical limit, which is calculated in 1.0 nMhe calibration curve was plotted from the variationate concentrations (at +0.5 V), which is presented ine sensitivity is calculated from the calibration curve,se to 1.667 A cm2M2. The linear dynamic rangebonate sensor displays from 1.0 nM to 1.0 mM [linearity,. 5D (inset)] and the detection limit was considered asnM [3 noise (N)/slope(S)]. Signal-to-noise ratio (often

SNR or S/N) is a measure used in science and engi-t compares the level of a desired signal to the level ofd noise. It is dened as the ratio of signal magnitude toagnitude, often expressed in decibels [4447].acteristic of the CuO-ZnO NRs/AgE is activated as a

bicarbonate concentration at room conditions, whereponse is observed to be maximum. As measured, theponse of the nanostructure-lms increases with theoncentration of target analytes. Similar phenomenaseen reported in the literature [4851]. For a low con-of bicarbonate in liquid medium, there is a smallererage of bicarbonate molecules on the NRs/AgE andrface reaction proceeds slowly. On an increase in bicar-centration, the surface reaction of NRs/AgE increasesge surface area contacted with bicarbonate molecules,

a gradually increase the response. On a further bicarbonate concentration of the CuO-ZnO NRs/AgE

has low-dimensional crystallite size and low-lattice

disorder, pthe much lCuO-ZnO Nof bicarbonsaturation, bicarbonateas follows. IZnO NRs/Agbuffer solu10.0 s for tsteady statecan be attrin NRs/AgEactivity, ansitivity of tlimit is comsensors, bato high speNRs/AgE) olyte detectiNRs/AgE prenhanced tof NRs andducibility aand effectivwhich can chemicals felds./AgE (in the presence of bicarbonate); (C) concentration variations of of LDR of calibration plot (inset).

resents a more rapid increase in response because ofarger surface coverage of bicarbonate chemical on theRs/AgE lm surface. Generally, the surface coverageate molecules on the NRs/AgE lms starts to achievewhich follows to a gradual increase in response. The-sensing mechanism of the NRs/AgE lm is explainednitially, oxygen (dissolved) is chemisorbed on the CuO-E surfaces, when the NRs lm is immersed in phosphatetion system. Actually the response time was aroundhe fabricated CuO-ZnO NRs/AgE to reach in saturated

system. The high sensitivity of the fabricated NRs/AgEibuted to the excellent absorption (porous surfaces) and adsorption ability, high catalytic-decompositiond good biocompatibility of the NRs. The estimated sen-he fabricated sensor is relatively higher and detectionparatively lower of new fabricated bicarbonate chemi-sed on doped nano-materials modied electrodes. Duecic surface area, the nanostructure materials (here,ffer a favourable nano-environment for the target ana-on with large quantity [5261]. The high sensitivity ofovides high-electron communication features, whichhe direct electron transfer between the active sites

at AgE. The fabricated NRs/AgE had a good repro-nd stability. The NRs/AgE system exemplied a simplee approach to the detection of bicarbonate chemicals,demonstrated the signicant access to a large group ofor wide range bio-medical applications in health-care


Fig. 6. Selectiv ) IV r+0.5 V (presen o +1.5

Oxygen while NR coing the chemspecies (succonductionbicarbonatesented in be

O2(diss) OO2(ads) + e

O2(ads) + e

NaHCO3(liq)

H2O +

Here, thence of carcarbonic acreacted witTherefore itcurrent) of time was arurated steaNRs/AgE cosurfaces in Ndecomposit

In liquimechanismbonate analexecuted aited semicodecreases utance is decincreasing tference (forpresence otbicarbonateNRs embedof all inferrdeducting t

By deducurrent valutone 4.6%; niodide 2.5%

ore, is bioroduatedment

witht restaine

weelly donrpliesamped tollec0 Llutioed (samped tlyzin

exhanoals [eries0.1 ME sesor-ity studied with various analytes using semiconductor CuO-ZnO nanomaterials. (Ated in percentage); analyte concentration was taken at 0.1 M. Potential range: 0 t

(dissolved) is chemisorbed onto the NRs/AgE surfaces,ated-lm electrode is immersed into PBS system. Dur-i-sorption, the dissolved oxygen is converted to ionic

h as O2 and O) which is gained electrons from the band of doped NRs. The reaction mechanism between

and ionic oxygen species adsorbed on NRs/AgE is pre-low in Eqs. (10)(12).

2(ads) (10)

O2(ads) (11) 2O(ads) (12)

+ nO(adsonNRs) HCO3 + Na+ + nO(ads) CO2 + NaOH + ne (13)

e reaction (Eq. (13)) is directly depended in the pres-bonic acid into reaction system. On NRs/AgE surfaces,id is oxidized to convert as bicarbonate and H+, which ish O(ads) to release electrons into the conduction band.

is decreased the resistance (increasing the conductionthe NRs lm embedded on AgE. Actually the responseound 10.0 s for the fabricated NRs/AgE to reach the sat-dy-state level. The higher sensitivity of the fabricateduld be attributed to the excellent absorption (porousRs/binders/AgE) and adsorption ability, high catalytic-ion activity, and good biocompatibility of the NRs.d phase, the dependent on reactant constituents,

Thereftowardthe repNRs/coexperioughlycurrenwas reto 2ndgradua

To cwas apwater was usples c(100.bulk sodisplaywater conrmfor anabonateusing nchemic

A sate in NRs/AgThe sen of dissociation, and further chemi-sorption of a bicar-ytes on the particular CuO-ZnO NRs/AgE surfaces werend explained in details later. The NRs/AgE is exhib-nductor behaviours, where the electrical resistancender the presence of bicarbonate agents. The lm resis-reased gradually (increasing the resultant current) uponhe bicarbonate concentration in solution phase. Inter-

selectivity) was studied for bicarbonate sensor in theher chemicals like ethanol, acetone, uoride, chloride,, nitrate, sulfate, and iodide using the doped CuO-ZnOded on at AgE (Fig. 6A). Current responses (at 0.5 V)ing analytes converted into percentile (% responses) byhe blank current are calculated and presented in Fig. 6B.cting the current value of blank solution, it is found thee is less than 5% for all chemicals (sulphide 3.9%; ace-itrate 4.4%; uoride (4.1%; ethanol 2.1%; methanol 3.7%;; blank 0%) compared to target bicarbonate (98.7%).

bonate deteTo investigthe NRs setime. The losor was inv

Table 1DeterminationZnO NRs/AgE concentration

Real sample

DI water Mineral watDistilled waTape water Sea water (Jesponses of various analytes and (B) current responses of analytes at V; delay time: 1.0 s.

t is clearly demonstrated the sensor is most selectivecarbonate compared with other chemicals. To checkcibly and storage stabilities, IV response for CuO-ZnO

AgE sensor was examined (up to 2 weeks). After each, the fabricated NRs/AgE substrate was washed thor-

the phosphate buffer solution and observed that theponse was not signicantly decreased. The sensitivityd almost same of initial sensitivity up to week (1stk), after that the response of the fabricated electrodeecreased.m the validity of the IV method, the CuO-ZnO NRs/AgEd to the determination of bicarbonate in various realles. In real sample study, a standard addition methodo estimate the accuracy of bicarbonate in water sam-ted from Jeddah in Saudi Arabia. The xed amount) of real water sample mixed and analyzed in 50.0 mLn by fabricated CuO-ZnO NRs/AgE electrodes. ResultsTable 1) that the detection of bicarbonate in groundles was reasonable for trace analysis and apparentlyhat the IV method is satisfactory, reliable and suitableg real samples with CuO-ZnO NRs/AgE systems. Bicar-ibited the maximum current response by IV systemmaterials fabricated AgE electrode compared to others6265].

of six successive measurements of 1.0 nM bicarbon- PBS yielded a good reproducible signal at CuO-ZnOnsor with a relative standard deviation (RSD) of 3.8%.to-sensor and run-to-run repeatability for 1.0 nM bicar-

ction were found to be 1.7% using CuO-ZnO NRs/AgE.ate the long-term storage stabilities, the response fornsor was determined with the respect to the storingng-term storing stability of the CuO-ZnO NRs/AgE sen-estigated signicantly. The sensitivity retained 93.0% of

of bicarbonate concentration at different real water samples by CuO-(amount of injection: 100.0 L; bulk solution: 50.0 mL; calibrated

range: 1.0 nM to 0.1 M).

s Measured currentat +1.0 V (A)

Respectiveconcentration (nM)

1.306 0.99 0.2er 1.315 0.99 0.2ter 1.345 3.5 0.2

1.437 4.3 0.2eddah, Saudi Arabia) 1.767 7.6 0.2


initial sensitivity for several days. The above results clearly sug-gested that the sensor can be used for several weeks without anysignicant loss in sensitivity [6570].

5. Conclus

Finally, sensor baseAgE with coprepared byous alkalineconomicalstudied by sthe analytidetection liwell as represearch acand bicarbonally, the results for asamples. Thas an effectusing CuO-Zorganized rdetection ofscale.

Acknowled

This wo(DSR), King144-D1435technical an

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we have successfully fabricated bicarbonate chemicald on low-dimensional CuO-ZnO NRs immobilized atnducting binders for the rst time. CuO-ZnO NRs are

wet-chemical method with reducing agents in aque-e system, which represents a simple, convenient, and

approach. CuO-ZnO NRs/AgE sensor for bicarbonate isimple IV method at room conditions and investigatedcal performances thoroughly in terms of sensitivity,mit, response time, selectivity, and storage stability asroducibility. The present work is provided extensivetivities that convened on the synthesis, characterizationnate sensing application of CuO-ZnO NRs using AgE. Theproposed IV method provided reasonable and reliable

selective determination of bicarbonate in ground waterus, the method may play an important role for using itive approach for a selective detection of bicarbonatenO NRs/AgE. This novel approach is introduced a well-oute of efcient chemical sensor development for the

bicarbonate chemicals in environmental elds in broad

gements

rk was funded by the Deanship of Scientic Research Abdulaziz University, Jeddah, under grant no. 130-. The authors, therefore, acknowledge with thanks DSRd nancial support.

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Biographies

Mohammed M. Rahman received his Ph.D. degree last 2007 on Electrochem-istry under the School of Natural Science, Chonbuk National University, Korea. Hehas successfully completed two years post-doctoral research fellowship in PusanNational University (2007/2008, South Korea) and Toyohashi University of Technol-ogy (2008/2009, Japan) respectively. Recently he completed two-year contract asan assistant professor in the chemistry department and CAMNE, Najran University,KSA (2009/2011). Presently, he is working as an assistant professor in the chemistrydepartment and CEAMR, King Abdulaziz University, KSA since 2011. His currentresearch interest is the development and potential applications of chemi-sensorsusing semiconductor doped and un-doped nanostructure materials.

Abdullah M. Asiri received Ph.D. from University of Wales, College of Cardiff, UK. He ise Octce foremistromicf texttics. H

is alsganic A). H

. sensor perspective, Environ. Sci. Technol. 41 (2007) 63336342.man, S.B. Khan, M. Faisal, M.A. Rub, A.O. Al-Youbi, A.M. Asiri, Determi-olmisartan medoxomil using hydrothermally prepared nanoparticlesd SnO2Co3O4 nanocubes in tablet dosage forms, Talanta 99 (2012)

, M.M. Rahman, K. Akhtar, A.M. Asiri, M.A. Rub, Nitrophenol chemi-d active solar photocatalyst based on spinel hetaerolite nanoparticles,

9 (2014) e85290.. Bian, Z. Du, C. Wang, Measurement and prediction model of car-de solubility in aqueous solutions containing bicarbonate anion, Fluidilib. 386 (2015) 5664.

in 1995sity sincExcellenPhotochphotochdyeing oand plasence. He(UK), Orence (USBranch) the Head of the Chemistry Department at King Abdulaziz Univer-ober 2009 and he is the founder and the Director of the Center of

Advanced Materials Research (CEAMR). He is a professor of Organicry. His research interest covers colour chemistry, synthesis of novel, thermochromic systems, synthesis of novel colouring matters andiles, materials chemistry, nanochemistry, nanotechnology, polymers,e is the Editor-in-Chief of King Abdulaziz University Journal of Sci-o a member of the Editorial Board of Pigments and Resin TechnologyChemistry in Sight (New Zealand), Recent Patents on Materials Sci-e is the Vice-President of Saudi Chemical Society (Western Province

Development of selective and sensitive bicarbonate chemical sensor based on wet-chemically prepared CuO-ZnO nanorods1 Introduction2 Experimental2.1 Materials and methods2.2 Synthesis and growth mechanism of CuO-ZnO NRs2.3 Fabrication and detection technique of bicarbonate

3 Results and discussion3.1 Analysis of optical and structural properties3.2 XPS analysis3.3 Morphological analysis3.4 Elemental analysis

4 Chemical sensor application4.1 Fabrication of bicarbonate sensor using CuO-ZnO NRs4.2 Detection of bicarbonate by CuO-ZnO NRs

5 ConclusionAcknowledgementsReferences

Biographies

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