Hyeong-U
Kim‡
a,
Sreekantha Reddy
Dugasani‡
ab,
Atul
Kulkarni
c,
Bramaramba
Gnapareddy
ab,
Jang Ah
Kim
a,
Sung Ha
Park
*ab and
Taesung
Kim
*ac
aSKKU Advanced Institute of Nano Technology (SAINT), Sungkyunkwan University, Suwon, Gyeonggi Do 440-746, South Korea. E-mail: tkim@skku.edu; sunghapark@skku.edu
bDepartment of Physics, Sungkyunkwan University, Suwon, Gyeonggi Do 440-746, South Korea
cSchool of Mechanical Engineering, Sungkyunkwan University, Suwon, Gyeonggi Do 440-746, South Korea
First published on 29th July 2015
In the present study, an optical fiber sensor using pristine DNA and metal ion-modified DNA (M-DNA) double-crossover (DX) lattices was fabricated for detecting volatile organic compounds (VOCs) in the range of their threshold limit values (TLVs). The selective incorporation of metal ions, such as cobalt ions (Co2+), with appropriate concentrations into the DNA DX lattice has to be considered since excess metal ions can be bound nonspecifically and degrade the functions of the M-DNA DX lattices. The peak intensity of light reflected from a Co2+-modified DNA (Co-DNA) DX lattice was observed to be about 32% greater than that from the pristine DNA DX lattice in the presence of a relatively high 11897 ppm concentration of the VOC methanol, and was also observed to be greater in the presence of 95 to 475 ppm methanol. The change in the intensity of the reflected light due to the VOC interaction with the pristine DNA and the Co-DNA lattices corresponded to the differences in surface morphology visualized by atomic force microscopy and the differences in intensities from Raman spectroscopy.
Structural DNA nanotechnology has influenced the construction of a wide range of artificially designed nanostructures with diverse geometries.9–13 DNA is considered to be one of most promising biomaterials to fabricate nanostructures due to its Watson–Crick base-pairing rules, and its ability to self-assemble.14 There are well-established mechanisms for the binding of DNA to functional groups, which allows DNA to be a useful host material to incorporate functional nano- and bio-materials, including ions, nanoparticles, and proteins. DNA molecules that can be effectively conjugated to other species and hence be used in various devices including sensors have been envisioned to have potential applications in electronics, spintronics, photonics, biosensors, and energy science.15–21 In particular, ions can be incorporated into DNA molecules in order to transfer their physical functionalities efficiently for practical use by a well-established methodology of M-DNA DX lattices without inducing structural deformations.22,23 With aid of this, the selective incorporation of metal ions with optimum concentration (without structural deformation) into a given DNA has to be considered because excess metals can bind non-specifically and degrade the functions of metal ion-modified DNA (M-DNA) double-crossover (DX) lattices. In the present study, we selected the divalent metal ion, Co2+ for use in the detection of volatile organic compounds (VOCs).
In order to enhance the sensitivity and efficiency of DNA as a sensor, we introduce the use of Co2+-modified DNA (Co-DNA) lattice for detecting VOC by measuring the change in the intensity of light reflected when the VOC interact with these lattices. Here, we constructed a reflection-mode sensor consisting of optical fibers as well as Co-DNA DX lattice for the sensing of various VOCs, such as methanol, dichloromethane (DCM), diethylamine (DEA), toluene, tetrahydrofuran (THF) and ammonia. The surface morphologies of the M-DNA lattices before and after exposure to these VOCs were studied by atomic force microscopy (AFM) and Raman spectroscopy. Finally we discuss the relationship between the change in the intensity of the reflected light and the change of the surface morphology of the Co-DNA DX lattice upon exposure to the VOCs.
DNA helices from each strand linked to helices from another strand at two crossover junctions; these linkages improve the stiffness of the structure (see Fig. S1, ESI†). The nucleotide sequence and shape of the DX tile was computer designed to impart on the tile a dimension of 12.5 × 6 nm2 (length × width) as shown in Fig. 2(a).25,26 Divalent Co2+ ions were intercalated into DNA DX lattice grown on the glass substrate via substrate-assisted growth (SAG) as depicted in Fig. 2(a) (enlarged view).27,28 Here, the nucleobases and phosphate backbones were the appropriate sites for the metal ions to bind to the DNA molecules (metal ion sites were indicated by red circles). We added metal ions to the DNA after formation of the DNA DX lattice in order to avoid structural deformation of the DNA DX lattice on the glass substrates. We noticed that 1 mM of Co2+ was optimally incorporated into the DNA without structural deformation. The AFM image (Fig. 2(b)) shows periodic DNA DX lattice with 1 mM Co2+. The periodicity in the DNA DX lattice is shown in the inset of Fig. 2(b) which is a noise-filtered 2D spectrum image obtained using the fast Fourier transform (FFT).
Various M-DNA (M = Co2+, Cu2+, Ni2+, Zn2+) DX lattices were first evaluated to determine their responses to various VOCs (Fig. S2, ESI†). Among them, Co-DNA DX lattices showed the best response to a VOC, which occurred when methanol was tested as the VOC. Hence, a further detailed evaluation was carried out for the Co-DNA DX lattice. The peak intensities of light reflected by pristine DNA and Co-DNA DX lattices in response to same concentration of various VOCs are shown in Fig. 3(a). A relatively high 11897 ppm concentration of the VOC was used in these experiments in order to obtain the strongest possible reflected intensities. From our observation, we realized that the reflected intensities were almost identical (after measuring multiple sets of the DNA and the Co-DNA lattices) at a given VOC concentration. This might be due to the high concentration of VOCs, which means VOCs were already over the saturation concentration to react with either the DNA or the Co-DNA lattices maximally. The wavelength chosen was based on tests of light sources with λ = 455, 530 and 660 nm; those with λ = 660 nm produced the most intense reflected light from the pristine DNA and Co-DNA DX lattices, respectively (see Fig. S3, ESI†).29 Consequently, we chose to use the light source with λ = 660 nm in these experiments. Of the various VOCs tested, the intensity of the light reflected from the lattices exposed to methanol was found to be the greatest, as seen in Fig. 3(a).
The intensity of light reflected by the Co-DNA DX lattice in response to methanol as well as by the pristine DNA lattice in response to methanol was then followed as a function of time elapsed after the injection of the methanol into the testing chamber, as shown in Fig. 3(b). On the one hand, pristine DNA started to respond to methanol sooner after the injection—that is, when the level of the methanol was still low—than did the Co-DNA DX lattice. This rapid response by pristine DNA may be due to the weak hydrogen bonds between its base pairs,30 and the delayed response to methanol by the Co-DNA DX lattice may be explained by Co2+ replacing OH− groups in this lattice and hence stabilizing its structure. On the other hand, the peak intensity of the reflected light in the presence of methanol was observed to be about 32% greater from the Co-DNA DX lattice than from the pristine DNA DX lattice. This enhanced sensitivity of the Co-DNA DX lattice to methanol at the later time points may also be explained by the stabilization of the lattice imparted by Co2+. In contrast to the case of pristine DNA, whose weak bonds would continue to be broken as more time elapses and the concentration of the methanol increases in the chamber, the stabilized Co-DNA DX lattice could continue to react with methanol. Overall, the Co-DNA DX lattice reflected light at higher intensities than did the pristine DNA DX lattice due to the presence of the Co2+ ions, which also caused a shift of the absorbed light to higher wavelengths. Furthermore, the appreciable binding of Co2+ to the intrinsically charged DNA molecules may increase the hydrophilicity of the lattice, which in turn might enhance the binding of VOCs to these Co-DNA DX lattice, thus resulting in the observed larger response.
We also tested various concentrations of methanol vapor to determine whether the sensing limits of the pristine DNA and Co-DNA DX lattices satisfy TLVs according to international chemical safety cards (ICSCs). Fig. 4(a) shows time courses of the peak intensities of light reflected from the pristine DNA and Co-DNA DX lattices in the presence of methanol concentrations ranging from 475 to 9.5 ppm. For each of the two lattices, the peak intensity did as expected decrease with decreasing methanol concentration in this range, with the peak intensity of the reflected light being particularly weak from each lattice in the presence of 9.5 ppm methanol. The peak intensities of light reflected from the pristine DNA and the Co-DNA DX lattices in the presence of various concentrations of methanol vapor are plotted in Fig. 4(b). These reflected light intensity values for 475 and 19 ppm of methanol from the pristine DNA lattice were 1.3 mV and 0.5 mV, respectively, and from the Co-DNA DX lattice were 3.2 mV and 1.6 mV, respectively, indicative of a greater intensity of the reflected light in the TLV limits provided by the addition of Co2+ to the DNA. In order to obtain reliable and reproducible data, we measured each sample of the DNA and the Co-DNA DX lattices more than five times for sensing methanol in the concentration range from 9.5 to 475 ppm shown in Fig. 4(b). Additionally we prepared about 5 sets of the DNA and the Co-DNA lattices for verifying the measurement repeatability and data consistency of samples affected by methanol. From them, we observed deviations of the reflected intensities with whole range of methanol concentration up to 8%. For references, we displayed and discussed the multiple measurements of the reflected intensities under 95 ppm of methanol through the DNA and the Co-DNA lattices in Fig. S4, ESI.†
Moreover, between 19 ppm and 475 ppm, the relationship between the change in the methanol concentration and the change in the intensity of the reflected light was more linear for the Co-DNA DX lattice than for the pristine DNA DX lattice (Fig. 4(b)). Taken together, the Co-DNA DX lattices showed an overall improved performance as compared with the DNA DX lattice.
To provide a physical basis for the relative intensities of the reflected light under the different conditions, we used AFM to analyze surface morphologies. AFM images of the pristine DNA and Co-DNA DX lattices before and after exposure to methanol vapor are displayed in Fig. 5(a). The topologies of the pristine DNA and Co-DNA DX lattices changed upon exposure to methanol, appearing to show a change from full to partial coverage. Such a change may have been caused by a methanol-induced dissociation of the DX lattices. These changes allow dissociation followed by significant attributions in reflected intensity. The topological change is eventually enhanced the reflected intensity of DNA and Co-DNA due to the surface coverage is reduced, the incident light to penetrate through the DNA and get reflected from the back mirror, hence explaining the increase in the reflected intensity upon exposure to methanol (Fig. S5, ESI†). To quantify surface coverage, we evaluated the surface vacancy area percentages, and found them to be between ∼16 to ∼20% for the pristine DNA and Co-DNA DX lattices.
Raman spectra of pristine DNA and Co-DNA DX lattices are shown in Fig. 5(b). The intensities of the specific bands corresponding to the nucleobases and phosphate backbones of the DNA (shown in Table S1 in ESI†) became reduced by up to ∼70% upon exposure to methanol vapor. Such a reduction may be explained by the relatively high polarity of this VOC affecting DNA bonds as described above. We noticed that the change of Raman intensity (ΔIRaman) corresponds to the change of vacancy area (ΔAvacancy) as shown in Fig. 5(c): based upon the experimental data obtained from the pristine DNA and Co-DNA DX lattices, we observed that when ΔIRaman is large, so is ΔAvacancy, which can be explained by the dissociation of DNA DX lattice on the given substrate after exposure to methanol vapor.
Footnotes |
† Electronic supplementary information (ESI) available: Schematics of DX structure, control experiments, and repeatability data. See DOI: 10.1039/c5ra11371g |
‡ These authors contributed equally to this work. |
This journal is © The Royal Society of Chemistry 2015 |