This content has been downloaded from IOPscience. Please scroll down to see the full text.

Download details:

IP Address: 169.235.8.49

This content was downloaded on 21/01/2014 at 17:42

Please note that terms and conditions apply.

Metal nanoparticles and DNA co-functionalized single-walled carbon nanotube gas sensors

View the table of contents for this issue, or go to the journal homepage for more

2013 Nanotechnology 24 505502

(http://iopscience.iop.org/0957-4484/24/50/505502)

Home Search Collections Journals About Contact us My IOPscience

IOP PUBLISHING NANOTECHNOLOGY

Nanotechnology 24 (2013) 505502 (11pp) doi:10.1088/0957-4484/24/50/505502

Metal nanoparticles and DNAco-functionalized single-walledcarbon nanotube gas sensors

Heng C Su1, Miluo Zhang1, Wayne Bosze1, Jae-Hong Lim2 andNosang V Myung1

1 Department of Chemical and Environmental Engineering, University of California-Riverside,Riverside, CA 92521, USA2 Electrochemistry Department, Korea Institute of Materials Science, Changwon 641-831, Korea

E-mail: myung@engr.ucr.edu

Received 10 May 2013, in final form 26 July 2013Published 27 November 2013Online at stacks.iop.org/Nano/24/505502

AbstractMetal/DNA/SWNT hybrid nanostructure-based gas sensor arrays were fabricated by means ofink jet printing of metal ion chelated DNA/SWNTs on microfabricated electrodes, followed byelectroless deposition to reduce metal ions to metal. DNA served as a dispersing agent toeffectively solubilize pristine SWNTs in water and as metal ion chelating centers for theformation of nanoparticles. Noble metals including palladium, platinum, and gold were usedbecause the high binding affinity toward specific analytes enhances the selectivity andsensitivity. The sensitivity and selectivity of the gas sensors toward various gases such as H2,H2S, NH3, and NO2 were determined at room temperature. Sensing results indicated theenhancement of the sensitivity and selectivity toward certain analytes by functionalizing withdifferent metal nanoparticles (e.g., Pd/DNA/SWNTs for H2 and H2S). The combinedresponses give a unique pattern or signature for each analyte by which the system can identifyand quantify an individual gas.

(Some figures may appear in colour only in the online journal)

1. Introduction

The electronic nose (E-nose) which simulates the humanolfactory system was advanced as a promising device toaccurately identify, differentiate, and quantify the presenceof hazardous gaseous chemicals to improve the quality oflives [1]. Over the years, several sensor configurations havebeen developed by detecting different changes of transducers’properties ranging from piezoelectric [2], which monitorthe frequency change of functionalized polymer coatedelectrodes when absorbing gaseous molecules, to chemire-sistive/conductometric sensors that generate the electricalresistance/conductance change upon gas adsorption/reactionon semiconducting sensing materials [3]. Compared to othersensor modes, chemiresistive metal oxide sensors drewthe attention of researchers because of its robust nature,simple circuit design, and low cost for a miniaturized and

field-deployable device. Despite the good sensitivity, fastresponse/recovery time, and long life time, metal-oxide-basedchemiresistors need to operate at high temperature whichultimately increases their power consumption. On the otherhand, short life-time, drift of resistance, and greater interfer-ence from humidity overshadow the advantages of conductingpolymer-based chemiresistors such as room temperatureoperation, good response to a wide range of analytes, andfast response/recovery time to polar gaseous compounds [4].Consequently, facile approaches to synthesizing a rapidresponse, reversible and selective gaseous chemical detectionsensor array at room temperature, which provide considerablebenefits in terms of low cost and manufacturability for massproduction, are essential for environmental monitoring.

Advances in nanotechnology have prompted the utiliza-tion of one-dimensional (1D) semiconducting nanostructuresas novel sensing and/or transducer materials due to their

10957-4484/13/505502+11$33.00 c© 2013 IOP Publishing Ltd Printed in the UK & the USA

Nanotechnology 24 (2013) 505502 H C Su et al

ultra-high surface area to volume ratio that enhances surfaceadsorptive capacity, tunable electrical properties via dopantswhich later control the sensing performance, and the abilityto form an ultra-high-density array in small dimensions. Inparticular, single-walled carbon nanotubes (SWNTs) are apromising 1D nanomaterial due to their excellent electronicproperties, and their thermal and chemical stability atroom temperature for the development of devices withminiaturized size, simplicity, reliability, and low cost [5–8].The unique properties of SWNTs have motivated researchersto focus on SWNT-based chemical sensors. Among them,the SWNT-based chemiresistor has been widely applied dueto its sensitivity to various gaseous analytes via chargeinteraction and its simple measurement set-up. PristineSWNTs show remarkable adsorptive ability due to their largesurface area and once they have interacted with NH3,NO2and aromatic compounds, the electric behavior is changeddepending on the exposing analytes; nevertheless, as a sensingmaterial, pristine SWNTs have drawbacks, specifically of lowsensitivity, lack of selectivity and long recovery which hinderits use in making an applicable gas sensor [9]. Numerousefforts have been devoted to surface functionalization ofSWNTs, covalently or noncovalently, to improve their sensingperformance, including sensitivity and selectivity [9]. Currentprogress has shown that sensitivity toward certain analytes isfurther enhanced by introducing functional groups [10, 11].Numerous synthetic routes have been reported for thepreparation of functionalized SWNTs; each provided varieddegrees of control of the size and distribution of thedecorated SWNTs. These approaches can be divided intothree general categories: (1) electrochemical and chemicalprocesses [12, 13]; (2) dispersion of nanoparticles on thesurface of SWNTs [14]; (3) physical deposition includingsputtering and e-beam evaporation [15]. Although a largevariety of the metal nanoparticle decorated carbon nanotubeshave been successfully synthesized via physical routes,expensive equipment and high power consumption are themajor concerns and disadvantages. Homogeneous, highlystable and highly concentrated dispersed SWNTs with similarparticle size distributions can be achieved by the dispersionof nanoparticles. However, special functionalization steps forSWNTs are needed, e.g., to chemically functionalize SWNTswith the thiol group (–SH) [14] to react with gold. Further,extra purification steps are needed, resulting in lower yield forthe final SWNTs.

The electrochemical process is a powerful techniquefor depositing a wide range of metal/metal oxide on thesurface of the SWNTs due to its capability of controlling sizeand composition tuning. These hybrid nanostructures havedemonstrated the capability of enhancing the selectivity andsensitivity toward specific target analytes such as Pt [15, 16]and Pd for H2 [13], Au for H2S [17] and elementalmercury vapor [18], and ZnO and SnO2 for H2S, xylene andVOCs [11, 19]. Instead of decorating inorganic compounds,conducting polymer, e.g., PEDOT-PSS or polyaniline (PANI),and polythiophene [20, 21], via chemical or electrochemicalprocesses functionalized to the surface of SWNTs has showngreat sensing performance.

Despite the remarkable electrical and mechanicalproperties that the SWNT possesses, poor solubility inaqueous and non-aqueous solution is a considerable challengein many applications requiring uniform dispersion suchas the field effect transistor and chemical and biologicalmanipulation in fundamental and practical research fields.Functionalized SWNTs with various functional groups suchas carboxylic and PABS groups were realized to enhancethe solubility of SWNTs as well as introducing structuraldefect sites, destroying the intrinsic mechanical and electricalproperties of intrinsic SWNTs. In order to maintain theintrinsic electrical properties as well as the structuralconfiguration, surfactants were introduced via noncovalentlydecorating in the solution to disperse carbon nanotubesinto smaller bundles or individual tubes. Pristine SWNThas been successfully solubilized in aqueous media byadsorption of sodium dodecyl sulfate (SDS) [22], sodiumcholate [23], sodium dodecylbenzene sulfonate (SDBS) [24],and polymer rapping [25]. In particular, deoxyribonucleicacid (DNA) revealed better dispersion efficiency comparedto other polymers [26]. This is owing to the π stackinginteraction which overcomes the van der Waals attractionenergy of 500 eV µm−1 [27] between each carbonnanotube resulting in separation from the large bundles [28].Besides, DNA, a readily available biopolymer, is anexcellent chelating/complexing medium for nanostructuresbecause its rich chemical functionality allows it to interactwith a variety of nanomaterials of interest. For decades,deposition of cobalt [12], copper [29], gold [30], nickel [12],palladium [31], platinum [32], and silver [33] on DNA hasbeen investigated and successfully fabricated. To the best ofour knowledge, metal nanoparticles decorated DNA/SWNThybrid structures (Pt/DNA/SWNTs) have been synthesized,but not for the purpose of gas sensing [34, 35].

In this work, a series of metal/DNA/SWNT-based hybridnanostructure sensor arrays were fabricated by ink jetprinting to pattern metal ion (e.g. Au, Pt, and Pd) chelatedDNA/SWNTs onto prefabricated microelectrodes followedby electroless reduction to form metal nanoparticles (NPs).DNA was selected because of its ability to serve both asa dispersion and chelating agent. Ink jet printing was usedto deliver metal ion chelated DNA/SWNTs to prefabricatedelectrodes because of (1) its ability to be purely additiveon top of the existing features or patterns where needed;(2) its flexibility regarding choice of structure design bysimply changing the software-based printer control system;(3) its ability to pattern organic materials such as biologicalmaterials (DNA/RNA or proteins) that are not compatiblewith the conventional photolithography alignment process;(4) its compatibility for bulk/wafer-scale printing with highrepeatability; and (5) its low cost [36]. The ability to tunethe SWNT gas sensing performance, including sensitivity andselectivity, has been demonstrated through control of the ratioof DNA and SWNTs with different metal depositions to formhybrid nanostructures. These decorated hybrid nanostructuredevices were exposed to four different gases: H2S,H2,NH3,and NO2. Sensing performance of these hybrid nanosensorsin terms of sensitivity and selectivity, as well as response and

2

Nanotechnology 24 (2013) 505502 H C Su et al

Figure 1. Optical images of (a) patterned electrodes on a silicon wafer, (b) an electrode array with 3 µm single gap.

recovery time, was correlated to the metal decorated on thehybrid nanostructures.

2. Experimental procedure

The individually addressable sensor array consists of 24microelectrodes with a 3 µm gap microfabricated usingphotolithography (figure 1). Thermally oxidized highly dopedp-type Si wafer (Ultrasil Corporation, Hayward, CA) witha 300 nm oxidation layer (grown by low pressure chemicalvapor deposition) was used as the substrate. First, a positivephotoresist AZ 5214-E (AZ Electronics Materials USACorporation, Somerville, NJ) was spin-coated on the substrateat a speed of 1000 RPM for 2 s followed by 3000 RPMfor 30 s, and then the substrate was placed on a hot plateat 110 ◦C for 5 min. The patterns were photolithographicallydefined onto the substrates by being exposed under UV witha 70 mJ cm−2 exposure dose and subsequently developed byimmersing it into a developer (AZ 400 K:DI water = 1:4,AZ Electronics Materials USA Corporation, Somerville, NJ)for 45 s. Afterwards, Ti (an adhesion layer) and Au filmswere e-beam deposited with film thicknesses of 20 nm and180 nm, respectively. The substrate was soaked in an acetonebath overnight for the lift-off process. Prior to experiments,these prefabricated microelectrodes were diced, cleaned withacetone and water, and blow-dried with ultra-high-purity N2gas for further use.

Figure 2 illustrates the experimental steps to fabricatemetal/DNA/SWNT-based gas sensors. Carbon nanotubesuspension inks were prepared by mixing commerciallyavailable as-prepared SWNTs (AP-SWNT, Caron solutionInc., Riverside, CA) and sodium salt salmon testes DNA(% G–C content = 41.2%, MW = 1.3× 106 approximately2000 bp from Sigma) in 1:1 ratio. Salmon DNA was selectedthroughout this work because it is relatively inexpensive,stable, and easy to obtain in large quantities. The resultingDNA/SWNT solution was sonicated for 90 min (temperaturewas maintained at 9 ◦C or less) until a uniform suspensionwas obtained. Then the suspension was centrifuged at13 200 RPM for 15 min and the supernatant was subsequentlycollected. The concentration of the SWNTs was determinedby a UV–vis Spectrophotometer (Beckman Coulter ModelDU R© 800) using a calibration curve at the wavelength of730 nm with calculated concentration independent of thesolution’s pH [37]. Afterwards, metal ion chelated DNAwrapped SWNT inks were made by mixing DNA/SWNT

ink and 1 mM of corresponding metal ion solution. In thiswork, palladium chloride (PdCl2, pH = 5, incubationtime of 6 h) for Pd, potassium tetrachloroplatinate (K2PtCl4,pH = 5, incubation time of 8 h) for Pt, and chloroauricacid (HAuCl4, pH = 5.3, incubation time of 6 h) for Auwere used to synthesize the metal ion chelated DNA/SWNThybrid structure. Finally, the resulting mixtures were desaltedby dialysis through a semipermeable membrane (molecularweight cut off 12–14 kD, Spectro/Por R©) against nanopurewater overnight.

Prior to the ink jet printing process, the prefabricatedmicroelectrodes were cleaned with oxygen plasma at 200 Wfor 2 min (Oxford Plasmalab 100/180) at a pressure of20 mTorr and then placed on the plate of a Fujifilm DimatixDMP 2831 piezoelectric ink jet printer. The dialyzed inkwas put into a 1 ml cartridge and inserted into the printer.Different jetting voltage and frequency conditions were tested.As shown in figure 3, one droplet instead of multiple dropletswas achieved by applying a voltage of 15 V at 10 kHz, whichshows a perfect round shape without a droplet tail and jettingdirected straight to the substrate. Dot patterns were printedonto the prefabricated microelectrodes from silicon-basednozzles at the applied voltage of 15 V and frequency of 10 kHzaccording to the preliminary result. The printing cycle wasset at 20 times to reach the required resistance range, and theclean cycle was fixed for every five prints to avoid cloggingand misalignment. Metal/DNA/SWNT hybrid nanostructureswere finally obtained by dipping the substrate into 2 ml ofaqueous electrolyte containing 2.5 g l−1 of dimethylaminoborane (DMAB) solution, 25 g l−1 of sodium citrate, and25 g l−1 of 85% lactic acid; the pH was adjusted to 7.4 withNH4OH [31]. After 4 min, the microelectrodes were gentlyrinsed with nanopure water and blow-dried with ultra-puritynitrogen gas.

The morphologies and compositions of themetal/DNA/SWNT hybrid nanostructures were investigatedusing field emission-scanning electron microscopy (FE-SEM,FEG-XL30, Philips), transmission electron microscopy(TEM, FEI-PHILIPS CM300) and energy-dispersive x-rayspectroscopy (EDS). The electrical resistances of these hybridnanostructures were determined by two-point measurementwith a Keithley 236 source measurement. Furthermore,the sensing performance of the metal/DNA/SWNT hybridnanostructures was studied by installing a sensing chipin a sealed homemade glass dome with a gas inlet andoutlet ports for gas flow and then clipping the chip to a

3

Nanotechnology 24 (2013) 505502 H C Su et al

Figure 2. (a) Schematic of the metal/DNA/SWNT hybrid nanosensor fabrication, (b) ink jet printing of SWNT ink, and (c) nanosensorarray configuration with a customized glass dome sensing cell with a gas inlet and outlet.

Figure 3. Droplet images for the default wave form. The satisfactory droplet was obtained at applied voltage 15 V and frequency of 10 kHz.

4

Nanotechnology 24 (2013) 505502 H C Su et al

Keithley 236 source measurement to obtain an electricalconnection (figure 2(c)). The applied voltage was fixed at1 V where the resistance was continuously recorded. Duringthe sensing measurement, the total gas flow rate was fixed at200 sccm and dry air was used as the carrier gas in all theexperiments. Finally, the configuration of the sensing systemwas designed to measure each sensor in series simultaneouslywith multiplexers.

3. Results and discussion

3.1. DNA/SWNT ink preparation

The presence of DNA promoted the dispersion of theas-prepared SWNTs in nanopure water (figures 4(a) and(b)) and the supernatant of DNA/SWNTs (figure 4(c))remained stable for one week. The dispersion ability ofthe as-prepared SWNTs increases with increasing DNAconcentration. As shown in figure 4(d), solubilized SWNTconcentration increased from 0.13 to 0.16 mg ml−1 whenthe ratio of DNA:SWNTs increased from 1:1 (w/w) to 10:1(w/w). When the ratio of DNA:SWNTs is fixed at 100:1(w/w), the soluble SWNTs concentration can reach up to0.23 mg ml−1 which is approximately 85% of the startingSWNTs. However, as the ratio of DNA:SWNTs increases,the excess DNA segments can provide more metal chelationsites preventing metal ions from binding to the DNA/SWNThybrid nanostructures. Therefore, the as-prepared SWNTs aredispersed with the equivalent weight of DNA in this work.

3.2. Ink jet printing of DNA/SWNT inks

Upon ink jet printing of DNA/SWNTs, the challenge comesfrom the coffee ring effect, which prohibits the uniformdistribution of the SWNTs. This phenomenon is due toinhomogeneous, uneven evaporation of a liquid. During thedrying step, attractive capillary forces facilitated particleaggregation toward the rim of the droplet, resulting in adense, ring-like deposition on the flat surface [38]. However,the formation of the coffee ring effect is still under debate.Unlike Deegan et al, who suggested that pinning of thecontact line of a droplet caused replenishing flow from thecenter, Sommer et al [39] indicated that this phenomenon isindependent of the pinning of droplet contact line, while Huand Larson [12] suggested that the formation of the coffeering deposition requires not only a pinned contact line butalso a suppression of Marangoni flow. Although there isno cure-all solution to reduce or eliminate the coffee ringeffect, several approaches have been proposed. Printing on aheated substrate can minimize the formation of coffee ringdeposition in some systems due to the enhancement of solventevaporation rate reducing material transfer to the contact line.In this work, the coffee ring effect was more pronouncedas the substrate temperature increased and multiple rimsformed. The use of a binary mixture of solvents demonstratesa uniformly printed pattern when one uses a solvent witha higher boiling point. The coffee stain shrinks due to theincreasing percentage of higher boiling point solvent near

Figure 4. Optical images of (a) as-prepared SWNT ink,(b) DNA/SWNT ink and (c) supernatant from (b), and(d) concentration of soluble SWNTs plotted as a function of salmontaste DNA concentration measured by UV–vis at the wavelength of700 nm.

the pinned contact line resulting in a slower evaporation rateand diminishing the outward convective flow [40]. Surfacetreatment of the substrate provides an alternative method forreducing the coffee stain deposition. Small et al [6] reportedthat as substrate hydrophobicity increases along with theformation of the coffee ring, the oxygen-plasma-treated SiO2surface exhibits a higher degree of hydrophilicity resulting inthe uniform distribution of the carbon nanotube network [11].In this work, we used the oxygen plasma treatment techniqueto enhance the hydrophilicity and minimize the coffee ringeffect. Figures 5(a) and (b) show the contact angles of thesilicon dioxide before and after oxygen plasma treatmentwhich were 40.5◦ and 17.3◦, respectively. As expected,the DNA/SWNT network dispersed better on the morehydrophilic surface while the thick and aggregated ringstructure of a dried droplet can be clearly seen on the untreatedand more hydrophobic SiO2 chip (figures 5(c) and (d)).

3.3. Metal/DNA/SWNT hybrid nanostructure fabrication

During the incubation in electrolyte containing metal ions,metal ions attached to DNA form complex structures, andact as efficient nucleation sites for the growth of the

5

Nanotechnology 24 (2013) 505502 H C Su et al

Figure 5. (a) Untreated SiO2 chip with the contact angle of 40.5◦; (b) oxygen plasma treated SiO2 chip (200 W, 2 min) with the contactangle of 17.3◦; (c) and (d) SEM images of (a) and (b) after drying under room temperature, respectively.

nano-crystalline particles [32]. The SEM observation of theDNA/SWNT network (figure 6) shows that DNA formedclusters instead of wrapping up SWNTs. After incubation,the dialyzed solution was printed via the piezoelectric inkjet printer on to the pretreated chip and dipped into thedimethylaminoborane (DMAB) solution for electrochemicalreduction of chelated metal ions. DMAB was selectedas the reducing agent because of its stability in anelectroless deposition bath over a wide range of pH andtemperature without self-decomposition [41]. The standardredox potentials for Au, Pt, Pd and DMAB (equations (1)–(4))are listed below, where the oxidation of DMAB provides threeelectrons to reduce the metal ion complexes bound on DNA toform metal [32, 42]:

AuCl−4(aq) + 3e−→ Au(s) + 4Cl−4(aq)Eo

= 1.00 V versus NHE (1)

[PtCl4]2−(aq) + 2e−→ Pt(s) + 4Cl−4(aq)E

o

= 0.76 V versus NHE (2)

Pd2+(aq) + 2e−→ Pd(s)E

o= 0.92 V versus NHE (3)

(CH3)2NHBH3 + 4OH−→ BO2−+

32 H2 + 2H2O

+ (CH3)2NH+ 3e−Eo= −1.18 V versus NHE. (4)

Figures 7(a)–(c) show TEM images of the metal/DNA/SWNT (Au (a), Pt (b), and Pd (c)) nanostructuresafter electroless deposition and figure 7(d) shows the averageparticle size of metal nanoparticles. In the TEM images,the long hollow tubes are SWNTs and the black sphericalparticles on top of them are the decorated nanoparticles, which

Figure 6. SEM image of the DNA/SWNT hybrid structurenetwork. The DNA formed as a bundle instead of wrapping aroundindividual SWNTs.

show similar morphology with similar coverage. The averagediameter was in the range of approximately 2–4 nm. Althoughthe undyed DNA cannot be seen in the TEM images, theEDS peaks of Na (small insets in the bottom of each figure)indicated the presence of DNA. In particular, the EDS resultsalso clearly show the corresponding metal peaks (Au, Pt, andPd, respectively); the Cu peaks were from the copper grid(inset of figure 7).

It is worth noting here that DNA, a readily availablebiopolymer, is an excellent chelating/complexing agent fornanostructures because its rich chemical functionality allowsit to interact with a variety of nanomaterials of interest,especially transition metals. Several review articles [43] havereported metal ions binding to the bases, to nucleosides,

6

Nanotechnology 24 (2013) 505502 H C Su et al

Figure 7. TEM images of (a) Au/DNA/SWNT hybrid nanostructures, (b) Pt/DNA/SWNT hybrid nanostructures, (c) Pd/DNA/SWNThybrid nanostructures; (d) average particle size (2–5 nm).

and to nucleotides of DNA. Sissoeff et al [44] reported thatmetal ions can be categorized into three classes accordingto the binding coordination sites. Those sites are base ketooxygen and ring nitrogen, phosphate oxygen, and sugarhydroxyl. In particular, negatively charged phosphate oxygenand ring nitrogen (N7 on the bases adenine and guaninewhile N3 on bases cytosine and thymine) are the favoritebinding sites for transition metal ions [43]. N7 binding sitesfor Pt(II) have been confirmed by Takahara et al [45] withan experiment for the anticancer drug cisplatin complex. Inaddition, an experiment on the interaction between DNA anddivalent cations conducted by Duguid et al [46] with Ramanspectroscopy also confirmed N7 as the favorite binding site forPt(II) and Pd(II) ions. In addition, the Pt NP size of 2 nm inDong’s work [35] which is in line with our results of 2.5 nm,provides further evidence that DNA has already wrapped ontoSWNTs.

3.4. Sensing performance of metal/DNA/SWNT hybridnanostructures

Figure 8 shows the sensing responses of the unfunctionalizedSWNT, DNA/SWNT and metal/DNA/SWNT (Au, Pt, Pd)hybrid nanostructures to H2,H2S,NH3, and NO2 at roomtemperature investigated with the customized sensing systemdescribed previously [13, 17]. These sensors were challengedwith a different concentration of analytes with 15 min

exposure and 20 min recovery times. The sensor response wasdetermined by the resistance change before and after exposureto the analyte, defined as (Rf−Ro)/Ro, and is shown in figure 8as a function of analyte concentration; Rf is the final resistanceat the peak height and Ro is the initial baseline resistance priorto analyte exposure. The response time is defined as the timefor the sensor to reach 90% of its steady-state value, and therecovery time is the time required for the sensor after exposureto return to 50% of its maximum response. In nearly all cases,metal/DNA/SWNT hybrid nanostructures improved responsetimes (for example, Pd/DNA/SWNT hybrid nanostructureswith a response time of 11 min for 20 ppmv H2S) withrespect to the bare SWNT networks (for example, with aresponse time of 14 min for 20 ppmv H2S). Moreover,the sensor responses of metal/DNA/SWNTs were furtherenhanced (for example, the normalized sensing response ofthe Pd/DNA/SWNT hybrid nanostructure of 1.65 can be seenin figure 8(b2) of the calibration curve toward H2S) withrespect to the bare SWNT networks which is about 0.4 whenexposed to H2S. Detailed information is listed in table 1 forall sensing results.

In particular, in the case of H2 and H2S, Pd/DNA/SWNThybrid nanostructures show supreme sensing performancefor all metrics with dramatically higher sensitivity andfaster response/recovery times, clearly superior to the otherdevices, figure 8(a1). Upon exposure to 100 ppmv H2 (lowest

7

Nanotechnology 24 (2013) 505502 H C Su et al

Figure 8. (a) Real-time sensing performance and (b) the calibration curve of ink jet printed SWNTs (orange), DNA/SWNT hybridnanostructures (red), Au/DNA/SWNT hybrid nanostructures (blue), Pt/DNA/SWNT hybrid nanostructures (green), and Pd/DNA/SWNThybrid nanostructures (magenta) to (1) N2, (2) H2S, (3) NH3 and (4) NO2.

concentration), a 25% change in electrical resistance wasachieved with response and recovery times of 13 min and6.1 min, respectively. Upon exposure to 2000 ppmv H2

(highest concentration), a 787% change in electrical resistancewas achieved with response and recovery times of 10 minand 1.6 min, respectively. The sensing mechanism of thePd/DNA/SWNT hybrid nanostructures toward H2 can beexplained by the alteration of the transport properties owing

to the interaction of adsorbed species to the Pd and SWNTsurfaces. The possible mechanism involves the adsorptionof the H2 onto the Pd surface, where the adsorbed atomichydrogen reacted with the O2 in the air and formed a hydroxylgroup. The hydroxyl group on the Pd surface will further reactwith the adsorbed hydrogen to form water [13]. Therefore,the overall reaction will be the oxidation of H2 to formH2O and release electrons into the SWNT network. This

8

Nanotechnology 24 (2013) 505502 H C Su et al

Figure 9. H2 sensing responses (a) before and (b) after electrochemical reduction.

Table 1. Comparison of the sensing performance of various metal/DNA/SWNT sensors to H2,H2S,NH3, and NO2. (Note: the responseand recovery time for H2,H2S,NH3, and NO2 were determined at 1000, 20, 50, and 5 ppmv, respectively, which are the personal exposurelevels for individual analytes except H2.)

StructuresSensitivity(% ppm−1)

Resp. time t90(min)

Rec. time t50(min)

Sensitivity(% ppm−1)

Resp. time t90(min)

Rec. time t50(min)

H2 H2S

SWNTs N/A N/A N/A 0.09 14 17DNA/SWNTs N/A N/A N/A 0.6 13.6 17Au/DNA/SWNTs N/A N/A N/A 2.23 12.5 12.7Pt/DNA/SWNTs 0.4 13 8.2 1.07 12.4 14Pd/DNA/SWNTs 0.5 10.6 1.6 10.98 10.9 7.15

NH3 NO2

SWNTs 0.16 12 8.9 −2 13 125DNA/SWNTs 0.2 13.4 10.1 −4.3 13 125Au/DNA/SWNTs 0.21 11.8 8.9 −6 10 12.6Pt/DNA/SWNTs 0.8 12 10 −10.78 13 14.7Pd/DNA/SWNTs 1.35 9.6 8.8 −9.4 11.2 13.3

causes the recombination of the electrons and holes in theSWNT network, and increases its resistance. Furthermore,Pd NPs on SWNTs serve as a nanogate electrode, whichscatters the carriers conducting through the SWNTs. Themagnitude of the resistance change increases along withthe number of Pd NPs [13]. As for the sensing mechanismof the Au/DNA/SWNT hybrid nanostructures, our previousresults of electrochemical deposited Au/SWNTs [17] indicatethat chemisorbed H2S decomposes to form Au–SH or Au–Sspecies on the Au NPs modulating the surface work functionof theAu nanoparticles, resulting in electron flow to SWNTs.This electron donating behavior caused the resistance ofAu/SWNTs to increase when exposed to H2S gas. In the caseof Pd, oxidation of H2S into SO2 occurred on the Pd NPsurface. H2S reacts with the surface-adsorbed oxygen speciesto form SO2 [47] and release the electron into DNA/SWNTscausing the increase of the resistance upon exposure of theH2S.

Control sensing experiments toward H2 for metalion/DNA/SWNT (Pt4+,Pd2+) hybrid nanostructures beforeand after reduction were conducted to prove that the sensingperformance only came from the metal NPs (Pt and Pd) andnot from its metal ions. In figure 9, as expected, the metalions/DNA/SWNTs demonstrate no response toward H2 before

reduction, confirming that the sensing response comes fromthe reaction of the analyte and metal NPs.

The sensitivities for each sensor to each analyte towardH2,H2S,NH3, and NO2 were calculated by taking theslope from the linear part of the sensing calibration curve,as compiled in figure 10. For H2 sensing, only twopositive responses from Pt/DNA/SWNT and Pd/DNA/SWNThybrid nanostructures are achieved, while the other threenanosensors barely responded. For H2S sensing, all of thefive nanosensors gave positive responses with a significantlyhigh response from Pd/DNA/SWNT hybrid nanostructures.For NH3 sensing, all of the five nanosensors exhibited positiveresponses as well but without any prominent response. ForNO2 sensing, all of the five nanosensors show negativeresponses and the responses were relatively high. Thus, thecombined responses give a unique pattern or signature foreach analyte by which the system can identify and quantifyan individual gas.

4. Conclusions

Metal/DNA/SWNT hybrid nanostructure-based nanosen-sors were synthesized by means of ink jet printing ofDNA/SWNTs followed by electroless deposition. The shape,

9

Nanotechnology 24 (2013) 505502 H C Su et al

Figure 10. Sensitivity of ink jet printed SWNT networks (orange),DNA/SWNT (red), and Au/DNA/SWNT hybrid nanostructures(blue), Pt/DNA/SWNT hybrid nanostructures (green), andPd/DNA/SWNT hybrid nanostructures (magenta) to NO2,NH3,H2S, and H2.

morphology and size of the metal nanoparticle particles werecontrolled by adjusting the electroless deposition conditionsand the precursor solution composition. Soluble pristineSWNT inks were obtained by wrapping SWNTs withDNA. With the contribution of the aromatic bases of DNAbackbone forming noncovalent π–π interaction bondingbetween the interface of DNA and SWNTs, negativelycharged DNA/SWNTs can be effectively dispersed in theaqueous solution due to electrostatic repulsion. Furthermore,the backbone of DNA also provides several binding sites formetal ions. Au, Pt, and Pd/DNA/SWNT hybrid nanostructureswere obtained by reducing the already bonded metal ions onDNA.

The sensitivity and selectivity of the gas sensors weredetermined by analyzing their response to various gases suchas H2,H2S,NH3, and NO2 at room temperature. Sensingresults indicated the enhancement of the sensitivity andselectivity toward certain analytes by being functionalizedwith different metal nanoparticles. The combined responsesgive a unique pattern or signature for each analyte by whichthe system can identify and quantify an individual gas.

Acknowledgments

We gratefully acknowledge the financial support fromthe Fundamental R&D Program for Core Technology ofMaterials funded by the Ministry of Knowledge Economy,Republic of Korea and Korean Institute of Materials Science(KIMS) academic lab program.

References

[1] Nagle H T, Gutierrez-Osuna R and Schuffman S S 1998 Thehow and why of electronic noses IEEE Spectr. 35 22–31

[2] King W H 1964 Piezoelectric sorption detector Anal. Chem.36 1735–9

[3] Choi N-J, Kwak J-H, Lim Y-T, Bahn T-H, Yun K-Y, Kim J-C,Huh J-S and Lee D-D 2005 Classification of chemical

warfare agents using thick film gas sensor array SensorsActuators B 108 298–304

[4] Arshak K, Moore E, Lyons G M, Harris J and Clifford S 2004A review of gas sensors employed in electronic noseapplications Sensor Rev. 24 181–98

[5] Artukovic E, Kaempgen M, Hecht D S, Roth S and Gruner G2005 Transparent and flexible carbon nanotube transistorsNano Lett. 5 757–60

[6] Kordas K, Mustonen T, Toth G, Jantunen H, Lajunen M,Soldano C, Talapatra S, Kar S, Vajtai R and Ajayan P M2006 Inkjet printing of electrically conductive patterns ofcarbon nanotubes Small 2 1021–5

[7] Sharma P and Ahuja P 2008 Recent advances in carbonnanotube-based electronics Mater. Res. Bull. 43 2517–26

[8] Roman C, Helbling T, Hierold C and Bhushan B 2010Springer Handbook of Nanotechnology ed B Bhushan(Berlin: Springer) pp 403–25

[9] Ting Z et al 2008 Recent progress in carbon nanotube-basedgas sensors Nanotechnology 19 332001

[10] Zhao B, Hu H and Haddon R C 2004 Synthesis and propertiesof a water-soluble single-walled carbon nanotube–poly(m-aminobenzene sulfonic acid) graft copolymer Adv.Funct. Mater. 14 71–6

[11] Song J-W, Kim J, Yoon Y-H, Choi B-S, Kim J-H and Han C-S2008 Inkjet printing of single-walled carbon nanotubes andelectrical characterization of the line patternNanotechnology 19 095702

[12] Gu Q, Cheng C, Suryanarayanan S, Dai K and Haynie D T2006 DNA-templated fabrication of nickel nanoclusterchains Physica E 33 92–8

[13] Mubeen S, Zhang T, Yoo B, Deshusses M A and Myung N V2007 Palladium nanoparticles decorated single-walledcarbon nanotube hydrogen sensor J. Phys. Chem. C111 6321–7

[14] Sainsbury T, Stolarczyk J and Fitzmaurice D 2005 Anexperimental and theoretical study of the self-assembly ofgold nanoparticles at the surface of functionalizedmultiwalled carbon nanotubes J. Phys. Chem. B109 16310–25

[15] Star A, Joshi V, Skarupo S, Thomas D and Gabriel J-C P 2006Gas sensor array based on metal-decorated carbonnanotubes J. Phys. Chem. B 110 21014–20

[16] Vairavapandian D, Vichchulada P and Lay M D 2008Preparation and modification of carbon nanotubes: reviewof recent advances and applications in catalysis and sensingAnal. Chim. Acta 626 119–29

[17] Mubeen S, Zhang T, Chartuprayoon N, Rheem Y,Mulchandani A, Myung N V and Deshusses M A 2009Sensitive detection of H2S using gold nanoparticledecorated single-walled carbon nanotubes Anal. Chem.82 250–7

[18] McNicholas T P, Zhao K, Yang C, Hernandez S C,Mulchandani A, Myung N V and Deshusses M A 2011Sensitive detection of elemental mercury vapor bygold-nanoparticle-decorated carbon nanotube sensorsJ. Phys. Chem. C 115 13927–31

[19] Hernandez S C, Hangarter C M, Mulchandani A andMyung N V 2012 Selective recognition of xylene isomersusing ZnO–SWNTs hybrid gas sensors Analyst137 2549–52

[20] Oliveira M M and Zarbin A J G 2008 Carbon nanotubesdecorated with both gold nanoparticles and polythiopheneJ. Phys. Chem. C 112 18783–6

[21] Yuana C L, Chang C P and Song Y 2011 Hazardous industrialgases identified using a novel polymer/MWNT compositeresistance sensor array Mater. Sci. Eng. B 176 821–9

[22] Yurekli K, Mitchell C A and Krishnamoorti R 2004Small-angle neutron scattering from surfactant-assistedaqueous dispersions of carbon nanotubes J. Am. Chem. Soc.126 9902–3

10

Nanotechnology 24 (2013) 505502 H C Su et al

[23] Lin S and Blankschtein D 2011 Role of the bile salt surfactantsodium cholate in enhancing the aqueous dispersionstability of single-walled carbon nanotubes: a moleculardynamics simulation study J. Phys. Chem. B 114 15616–25

[24] Moore V C, Strano M S, Haroz E H, Hauge R H, Smalley R E,Schmidt J and Talmon Y 2003 Individually suspendedsingle-walled carbon nanotubes in various surfactants NanoLett. 3 1379–82

[25] O’Connell M J, Boul P, Ericson L M, Huffman C, Wang Y,Haroz E, Kuper C, Tour J, Ausman K D and Smalley R E2001 Reversible water-solubilization of single-walledcarbon nanotubes by polymer wrapping Chem. Phys. Lett.342 265–71

[26] Zhang M, Jagota A, Semke E D, Diner B A, Mclean R S,Lustig S R, Richardson R E and Tassi N G 2003DNA-assisted dispersion and separation of carbonnanotubes Nature Mater. 2 338–42

[27] Girifalco L A, Hodak M and Lee R S 2000 Carbon nanotubes,buckyballs, ropes, and a universal graphitic potential Phys.Rev. B 62 13104–10

[28] Johnson R R, Johnson A T C and Klein M L 2007 Probing thestructure of DNA–carbon nanotube hybrids with moleculardynamics Nano Lett. 8 69–75

[29] Monson C F and Woolley A T 2003 DNA-templatedconstruction of copper nanowires Nano Lett. 3 359–63

[30] Kim H J, Roh Y, Kim S K and Hong B 2009 Fabrication andcharacterization of DNA-templated conductive goldnanoparticle chains J. Appl. Phys. 105 074302

[31] Richter J, Seidel R, Kirsch R, Mertig M, Pompe W, Plaschke Jand Schackert H K 2000 Nanoscale palladium metallizationof DNA Adv. Mater. 12 507–10

[32] Seidel R, Ciacchi L C, Weigel M, Pompe W and Mertig M2004 Synthesis of platinum cluster chains on DNAtemplates: conditions for a template-controlled clustergrowth J. Phys. Chem. B 108 10801–11

[33] Braun E, Eichen Y, Sivan U and Ben-Yoseph G 1998DNA-templated assembly and electrode attachment of aconducting silver wire Nature 391 775

[34] Wang X et al 2006 Carbon nanotube–DNA nanoarchitecturesand electronic functionality Small 2 1356–65

[35] Dong L 2009 DNA-templated synthesis of Pt nanoparticles onsingle-walled carbon nanotubes Nanotechnology 20 465602

[36] Park J-U, Lee S, Unarunotai S, Sun Y, Dunham S, Song T,Ferreira P M, Alleyene A G, Paik U and Rogers J A 2010Nanoscale, electrified liquid jets for high-resolutionprinting of charge Nano Lett. 10 584–91

[37] Zhao W, Gao Y, Brook M A and Li Y 2006 Wrappingsingle-walled carbon nanotubes with long single-strandedDNA molecules produced by rolling circle amplificationChem. Commun. 2006 3582–4

[38] Deegan R D, Bakajin O, Dupont T F, Huber G, Nagel S R andWitten T A 2000 Contact line deposits in an evaporatingdrop Phys. Rev. E 62 756–65

[39] Sommer A P and Rozlosnik N 2005 Formation of crystallinering patterns on extremely hydrophobic supersmoothsubstrates: extension of ring formation paradigms Cryst.Growth Des. 5 551–7

[40] Park J and Moon J 2006 Control of colloidal particle depositpatterns within picoliter droplets ejected by ink-jet printingLangmuir 22 3506–13

[41] Yamauchi Y, Momma T, Yokoshima T, Kuroda K andOsaka T 2005 Highly ordered mesostructured Ni particlesprepared from lyotropic liquid crystals by electrolessdeposition: the effect of reducing agents on the ordering ofmesostructure J. Mater. Chem. 15 1987–94

[42] Bard A J and Faulkner L R 2001 Electrochemical Methods:Fundamentals and Applications (New York: Wiley)

[43] Saenger W 1984 Principles of Nucleic Acid Structure(New York: Springer)

[44] Sissoeff I, Grisvard J and Guille E 1978 Studies on metalions–DNA interactions: specific behaviour of reiterativeDNA sequences Prog. Biophys. Mol. Biol. 31 165–99

[45] Takahara P M, Rosenzweig A C, Frederick C A andLippard S J 1995 Crystal structure of double-stranded DNAcontaining the major adduct of the anticancer drug cisplatinNature 377 649–52

[46] Duguid J, Bloomfield V A, Benevides J and Thomas G J Jr1993 Raman spectroscopy of DNA–metal complexes. I.Interactions and conformational effects of the divalentcations: Mg, Ca, Sr, Ba, Mn, Co, Ni, Cu, Pd, and CdBiophys. J. 65 1916–28

[47] Li H, Xu J, Zhu Y, Chen X and Xiang Q 2010 Enhanced gassensing by assembling Pd nanoparticles onto the surface ofSnO2 nanowires Talanta 82 458–63

11