Capillary-Driven Sensor Fabrication of Polydiacetylene-on-Silica Plate in 30 Seconds: Facile Utilization of π-Monomers with C18- to C25-Long Alkyl Chain.

By utilizing the capillary-force-driven action, a novel polydiacetylene-based sensor on the porous silica plate was developed within 30 s for π-diacetylene monomers with variable chain lengths. This method enables one to utilize diacetylene monomers even with the shorter alkyl chain length of C18-C21, which has not been possible with conventional methods. The invented sensor platform employing shorter monomers was found to perform better, as was demonstrated for gaseous and aqueous analytes, i.e., ammonia gas and nucleic acids in aqueous phase. This new polydiacetylene platform opens up the development of quick and easy fabrication and the use of chemical and biochemical chips.

Introduction

π-Conjugated polydiacetylenes have attracted great interest as component of sensors by virtue of their bichromatic properties. These polymers are formed via a 1,4-addition reaction upon exposure to UV light and undergo a transition from a blue, nonfluorescent form to a red, fluorescent form in response to structural changes induced by external stimuli.[1−6] Thereby, functionalization of the surface of polydiacetylene with molecular receptors has been used to develop label-free biological/chemical sensors for the detection of influenza virus, toxin, oligonucleotides, antigens, proteins, and bacteria.[7−15] These sensors have taken conventional forms of Langmuir–Blodgett thin films, bilayer vesicles, and two-dimensional array chips. However, they generally take a long time to fabricate, from a few hours to a few days. On materials selection aspect, diacetylenes with shorter alkyl chains are expected to be more sensitive to external perturbations,[16−19] with polydiacetylene with shorter alkyl chains requiring less thermal energy to disturb the organized structure, whereas more thermal energy is required in the case of long alkyl chains.[18,19] However, bilayer or multilayer structures made of diacetylene monomers having shorter (less than C23) alkyl chains often become unstable in aqueous solutions.[16] To overcome such limitations in the selection of monomers, we propose a direct and simple solution of utilizing silica plates typically used for the separation of nonvolatile mixtures by thin-layer chromatography (TLC).[20,21] In this study, we developed a capillary-force-driven adsorption process on silica surface that works successfully for diactylene monomers of variable chain length, especially those having shorter alkyl chains that are more sensitive to external perturbations. The resulting hydrophobic polydiacetylene-on-silica (polydiacetylene/silica) plate is investigated as a novel diagnostic sensor platform for both chemical gaseous and biomolecular analytes.

Results and Discussion

We aimed at developing a novel label-free sensor by allowing a solution of diacetylene monomers as the mobile phase to be drawn up through a porous silica plate by capillary action. A schematic diagram of the adsorption of diacetylene monomers on a silica plate is shown in Scheme a. A bare silica plate was dipped into a solution of diacetylene monomers dissolved in tetrahydrofuran (THF), and the solution was drawn up spontaneously through the plate by capillary-force-driven flow, resulting in a uniform adsorption of diacetylene on the silica plate within 30 s, as shown in Scheme a. The diacetylene/silica plate was exposed to 254 nm UV light for 30 s, and the blue-phase polydiacetylene/silica plate turned red-phase plate by external perturbation, as shown in Scheme b. Four types of diacetylene monomers with different alkyl chains are used (Scheme c). 10,12-Pentacosadiynoic acid (PCDA; 25 carbon atoms, C25), 10,12-tricosadiynoic acid (TCDA; 23 carbon atoms, C23), 10,12-heneicosadiynoic acid (HCDA; 21 carbon atoms, C21), and 10,12-octadecadiynoic acid (ODDA; 18 carbon atoms, C18) comprise the same carboxylic acid head group but different alkyl chain length. Bilayer vesicle structures made of monomers having same as or shorter alkyl chains than HCDA are unstable in aqueous solutions.[16] Among the monomers tested in the present study, ODDA having 18 carbon atoms is the shortest diacetylene monomer. The surface morphology of silica plates changed by the physisorption of PCDA (C25) monomer, as observed with the scanning electron microscopy (SEM), as shown in Figure a. The contact angle of the bare silica plate could not be measured because its surface is truly porous. A distinct increase in the roughness was observed when the concentration of the monomer solution was 200 μg/μL or higher. The change in wettability following physisorption of PCDA monomer is associated with the extent of hydrophobicity related with the formation of larger rough domains at concentrations of 200–500 μg/μL. The choice of diacetylene monomers out of C18–C25 also alters the surface morphology, as shown in Figure b. Surfactants with different alkyl chain lengths have been used to study the effect of size and morphological control of micro/nanostructures.[22−24] The cases of C21–C25 monomers formed larger domains, whereas those of C18 monomer observed smaller domains. Capillary-driven method also has the advantage, that a uniform coverage of diacetylene domains was formed compared to that of samples just dropping the diacetylene solution onto the glass surface, as shown in Figure S1. The real-time polymerization of PCDA (C25)/silica plate is shown in Movie S1, Supporting Information. Uniform polymerization was identified by every diacetylene derivative adsorbed on the silica plate, with blue/dark blue derivatives at a concentration of 500 μg/μL, as shown in Figure b. Concentration-dependent polymerization of various diacetylene/silica plates at 100–500 μg/μL is also shown in Figure S2. Blue/dark blue is expected to be related to the density of polymerizable domains. The larger the domain size of polydiacetylene adsorbed on silica plate, the darker the blue. A large domain means that a large number of diacetylene molecules were adsorbed on the silica plate. However, ODDA (C18) molecules were less adsorbed by utilizing capillary-driven force and formed less domain comparison with the case forming relatively large domain on glass substrate just dropping molecules (Figure S1). An exact understanding of why ODDA (C18) molecules were less adsorbed by utilizing capillary-driven force will be constructed in the further research.

Scheme 1

Schematic Illustration and Chemical Structures

Schematic illustration and chemical structures of (a) the adsorption of diacetylene monomers on a silica plate and (b) polydiacetylene/silica-based sensor. (c) Chemical structures of diacetylene monomers used.

Figure 1

(a) SEM images (upper) showing the surface morphology of silica plates and bright-field image about the contact angle (lower) with adsorbed PCDA (C25) monomers at various concentrations from 20 to 500 μg/μL. The surface morphology of the silica plate obviously changed when the PCDA monomer concentration was 200 μg/μL or higher. (b) SEM images (left) showing the surface morphology and bright-field images (right) after polymerization of silica plates adsorbing various adsorbed diacetylene monomers (500 μg/μL): (i) PCDA (C25)/silica plate, (ii) TCDA (C23)/silica plate, (iii) HCDA (C21)/silica plate, and (iv) ODDA (C18)/silica plate. Scale bars: 100 μm.

The wettability of silica plates was changed by the physisorption of diacetylene monomers at a concentration of 500 μg/μL, as shown in Table . Upon adsorption of diacetylene, the surface became hydrophobic. The contact angles of silica plates with adsorbed PCDA (C25), TCDA (C23), HCDA (C21), and ODDA (C21) were 150, 145, 120, and 80°, respectively. The hydrophobicity of the surface is related to the altered surface morphology, as observed in Figure b.

Table 1

Contact Angle of Silica Plates Adsorbing Diacetylene Monomers

The attenuated total reflection Fourier transform infrared (ATR-FTIR) spectra of polydiacetylene/silica plates arising from polymerization are shown in Figure a, and peak assignments are listed in Table . Unlike the spectrum of the bare silica plate, the spectra of the polydiacetylene/silica plates have methylene stretching bands at 2921–2919 and 2848–2846 cm–1, as well as a hydrogen-bonded carbonyl stretching band at 1693–1690 cm–1, which is consistent with the previously reported results.[16] The retention of these peak positions in our spectra indicates that the diacetylene monomers were well adsorbed on the silica plates via capillary action. It is expected that the intensity of TCDA (C23) and HCDA (C21) of FTIR was stronger than that of ODDA (C18). Molar concentration at the concentration of 500 μg/μL of PCDA (C25), TCDA (C23), HCDA (C21), and ODDA (C21) is 1.33, 1.44, 1.56, and 1.80 M, respectively. The number of ODDA molecules is the highest in this experiment. However, ODDA (C18) molecules were less adsorbed by utilizing capillary-driven force and formed less domain comparison with the case forming relatively large domain on glass substrate just dropping molecules (Figure S1). PCDA (C25) with the minimum number of molecules has a relatively weak peak intensity. A detailed understanding will be constructed in the further research. To investigate the thermochromic properties of diacetylene/silica plates, polymerized diacetylene/silica plates were gradually heated while fluorescence was monitored using a fluorescence microscope. The quantitative fluorescence obtained from the fluorescent images of polydiacetylene/silica plates at different temperatures is shown in Figure b. The annealing temperature was increased gradually from 25 °C, and the PCDA/silica plate started to emit fluorescence at ∼65 °C. In contrast, the silica plate with adsorbed ODDA (C18), which has a shorter alkyl chain length than PCDA (C25), showed a much more sensitive response to heat and started to emit fluorescence at ∼40 °C. This difference in sensitivity is related to the difference in alkyl chain length. Generally, less thermal energy is required to disturb the organized structure of polydiacetylene with shorter alkyl chains, whereas more thermal energy is required in the case of long alkyl chains.[18,19] The ATR-FTIR spectra and thermochromic properties of polydiacetylene/silica plates indicate the successful fabrication of a more sensitive polydiacetylene-on-silica plate that also preserves the intrinsic optical characteristics of polydiacetylene.

Figure 2

(a) ATR-FTIR spectra of silica plates with various adsorbed diacetylene monomers. Asymmetric CH2 bond (circle), symmetric CH2 bond (triangle), stretching C=O bond (rectangle), and scissoring CH2 bond (rhombus). The spectrum of the bare silica plate has no peaks, whereas the spectra of the diacetylene/silica plates have CH2 bond and C=O bond peaks. (b) Thermochromism of silica plates with adsorbed diacetylene monomers. The transition temperatures to emit fluorescence are ∼65 °C (PCDA (C25)/silica plate), ∼60 °C (TCDA (C23)/silica plate), ∼55 °C (HCDA (C21)/silica plate), and ∼40 °C (ODDA (C18)/silica plate), respectively.

Table 2

Peak Assignments of Silica Plates Adsorbing Diacetylene Monomers

sample	νaCH2 (cm–1)	νsCH2 (cm–1)	νC=O (cm–1)	δsCH2 (cm–1)
PCDA (C25)/silica plate	2919	2846	1690	1460
TCDA (C23)/silica plate	2919	2847	1691	1462
HCDA (C21)/silica plate	2921	2848	1693	1464
ODDA (C18)/silica plate	2921	2848	1693	1465

We applied this system first to the detection of ammonia moiety in the gaseous phase; fluorescence intensities and ATR-FTIR spectra after reaction with ammonia gas are shown in Figure . The fluorescence images of polydiacetylene/silica plates after reaction with ammonia gas are shown in Figure S3a. Interestingly, red fluorescence was strongly emitted by the ODDA (C18)/silica plate, which has shorter alkyl chains than the other plates, as shown in Figure a. Unfortunately, the PCDA (C25)/silica plate emitted relatively less fluorescence. In all of the ATR-FTIR spectra, the carbonyl band at 1693–1690 cm–1 was replaced by a carboxylic acid band at 1546–1545 cm–1, as shown in Figure b. This indicates that the peak shift is caused by ionic interactions between the carbonyl group of diacetylene and the amine group of ammonia. These results suggest that the interaction with ammonia resulted commonly in the chemical modification of the diacetylene’s head groups; however, there occurs insufficient energy to perturb the organized structure of polydiacetylene based on PCDA monomer having long alkyl chains.[18,19] Capillary-driven fabrication method also had the advantage of the ability to detect ammonia gas compared to samples just dropping the diacetylene solution onto the glass surface, as shown in Figure S3b,c. The capillary-driven polydiacetylene/silica plates having a uniform coverage of domains showed a stronger response than polydiacetylene just dropped on glass substrate.

Figure 3

(a) Quantitative fluorescence intensity of silica plates with adsorbed diacetylene monomers after reaction with ammonia gas (inset: scheme of blue-to-red transition of polydiacetylene-on-silica). (b) ATR-FTIR spectra of silica plates with adsorbed diacetylene monomers before (solid line) and after (dashed line) reaction with ammonia gas. Peaks were shifted from 1693–1690 to 1546–1545 cm–1 in all of the cases after reaction with ammonia gas.

In addition, a “hydrophobic” PCDA (C25)/silica plate enabled formation of liquid drop arrays containing aqueous solutions with pH values of ∼1 and ∼13, as shown in Movie S2, Supporting Information. The blue-to-red transition was immediately observed with dropping of high-pH solution, i.e., sodium hydroxide solution. Previous studies have established that the chromatic change upon adjusting the pH values can be attributed to the repulsive Coulombic interactions during surface ionization, which force adjacent chains apart and trigger a conformational change.[25−27] The acid–base properties of end group at the diacetylene monomers are confirmed to be expected for spatially confined interfacial carboxyl groups exhibiting attenuated acidity. In addition, the blue-to-red transition was related to the extent of deprotonation of the interfacial carboxyl groups, which accounts for the occurrence of the blue-to-red response triggered by the high pH range.

This novel hydrophobic polydiacetylene-on-silica system is then applied to label-free DNA detection experiments, as shown in Figure . A schematic diagram of the formation of aqueous liquid droplet arrays on the ODDA (C18)/silica and the successive hybridization of target DNA within a sessile drop lying on the plate is shown in Figure a. The probe DNA solution was applied to the ODDA/silica plate that had been treated with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide and N-hydroxysuccinimide (EDC/NHS). After exposure to 254 nm UV light, target DNA solution was applied to the plate. Hybridization of immobilized probe DNA with target DNA induces a phase transition of polydiacetylene from blue to red. By wisely utilizing the more sensitive ODDA/silica plate, we were able to detect target DNA within 30 min. Fluorescence images of ODDA/silica plate after reaction with DNA on an overall surface area are shown in Figure S4. The highest level of red fluorescence was emitted when the ODDA/silica plate was reacted with a solution of perfectly matched DNA, whereas little fluorescence was emitted when the plate was reacted with a random DNA sequence, as shown in Figure b. This indicates that complementary hybridization of probe and target DNA induced a change in the π-conjugation length of the polydiacetylene backbone, resulting in fluorescence emission. The fluorescence of the ODDA/silica plate following hybridization of target DNA was also examined as a function of the concentration of target DNA, as shown in Figure c. The target DNA was found to be detectable label-free up to a concentration of 1 nM scale. As the concentration of target DNA increased, the relative fluorescence intensity also increased, suggesting that the amount of hybridization far more strongly evoked the change in the π-conjugation length of the ODDA-based polydiacetylene backbone.

Figure 4

(a) Schematic diagram of the formation of aqueous liquid drop arrays on the ODDA (C18)/silica and the successive hybridization of target DNA on a diacetylene/silica plate. Quantitative fluorescence intensity of the ODDA/silica plate (b) after reaction with target DNA and random DNA both at 500 nM, (c) after reaction with various concentrations of target DNA ranging from 500 nM to 500 pM. The following DNA sequences were used: NH2-5′-ATCCTTATCAATATTTAACAATAATCC-3′ for probe DNA, 3′-TAGGAATAGTTATAAATTGTTATTAGG-5′ for target DNA, and 3′-AAAAAAAAAA-5′ for random DNA.

Conclusions

In summary, diacetylene/silica plates were fabricated via capillary-driven-force within 30 s. This quick and easy method enabled one to utilize diacetylene monomers with long (C25) to shorter (C18) alkyl chain length. FTIR spectroscopy, electron and fluorescence microscopies, and thermochromics analyses confirmed that this hybrid system retained the intrinsic optical function of polydiacetylene and showed better sensitivity against external stimulus when shorter chain monomer was adopted. Hydrophobic polymerized ODDA (C18)-on-silica plate was found to be much more capable of detecting ammonia gas and nucleic acids in liquid drop arrays. We believe that this new polydiacetylene platform fabricated on a porous silica plate finds on-site applications where simple and immediate treatments need to be met.

Experimental Section

Materials

10,12-Pentacosadiynoic acid (PCDA, C25), 10,12-tricosadiynoic acid (TCDA, C23), 10,12-heneicosadiynoic acid (HCDA, C21), and 10,12-octadecadiynoic acid (ODDA, C18) were purchased from GFS Chemicals. 2-(N-morpholino)ethanesulfonic acid (MES), 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC), N-hydroxysuccinimide (NHS), tetrahydrofuran (THF), and ammonia were purchased from Sigma-Aldrich. Silica-coated thin-layer chromatography (TLC) plate named TLC Silica gel 60 F254 glassplate was purchased from Merck, Germany. Oligonucleotides were synthesized by Bioneer, Korea. The anthrax lethal factor sequence was used as probe DNA: the anthrax lethal factor sequence was used as probe DNA: NH2-5′-ATCCTTATCAATATTTAACAATAATCC-3′. The target DNA sequence was 3′-TAGGAATAGTTATAAATTGTTATTAGG-5′, and the random DNA sequence (control) was 3′-AAAAAAAAAA-5′.

Preparation of Silica Plates Coated with Diacetylene Monomers

Diacetylene monomers were dissolved in THF at concentrations ranging from 20 to 500 μg/μL. Silica-coated TLC plates were dipped into diacetylene-THF solution, and migration of the solution by capillary action resulted in the adsorption of diacetylene on the silica plates within 30 s. The plates were removed from their solutions and dried at room temperature.

Thermochromism of Diacetylene/Silica Plates

Diacetylene/silica plates were exposed to 254 nm UV light for 30 s. The plates were heated on a hot plate and fluorescence was analyzed in situ. Real-time temperature was monitored with a thermometer.

Reaction of Ammonia Gas with Diacetylene/Silica Plates

A chamber was flushed with N2 gas, and then a clean petri dish containing 1 drop of ammonia solution was set in the chamber. Diacetylene/silica plates were exposed to 254 nm UV light for 30 s and took place in the chamber immediately after exposure to UV light and left there for 30 min. The concentration of ammonia gas in the chamber, measured with an analogue gas detector (GASTEC), was ∼500 ppm.

Surface Treatment and Reaction of DNA with the ODDA (C18)/Silica Plate

NHS (2.0 mM) and EDC (4.0 mM) were dissolved in MES buffer (0.2 M, pH 6). The prepared NHS/EDC solution was reacted with an ODDA/silica plate for 30 min to allow NHS to react with the carboxylic acids. A solution of NH2-functionalized probe DNA in deionized (DI) water (500 nM) was reacted with the NHS-treated ODDA/silica plate for 30 min, and then the plate was rinsed with DI water. Polymerization was initiated by exposing the plate to 254 nm UV light for 30 s. Target and random DNA were also reacted with plates for 30 min and rinsed with DI water.

Instrumentation

The surface morphologies were analyzed by field-emission scanning electron microscopy (S-4300, Hitachi, Japan). Prior to SEM analysis, diacetylene/silica plates were coated with platinum for 60 s. Surface wettability was measured using a contact angle meter (CA-DT, Kyowa, Japan). A compact UV lamp (UVG-11, UVP) was used for polymerization of diacetylene. Fluorescence microscopy measurements were made using a reflected fluorescence system (BX 51, Olympus, Japan) equipped with a mercury lamp. Fluorescence images were captured after excitation by 510–550 nm waves. The filters used ranged from 590 nm infrared waves. Quantitative fluorescence values were measured using Adobe Photoshop CS3. A Spectrum GX1 (PerkinElmer) was used to acquire ATR-FTIR spectra for the diacetylene/silica plates. A mercury–cadmium–telluride detector cooled with liquid nitrogen was used, and 512 scans at a resolution of 4 cm–1 were accumulated to obtain a reasonable signal-to-noise ratio.