N-rich porous organic polymer with suitable donor–donor–acceptor functionality for the sensing of nucleic acid bases and CO₂ storage application

Abstract

A new high surface area porous organic polymer PDVTD-1 (poly-divinylbenzene-co-tartardiamide) has been synthesized via radical copolymerization of divinylbenzene and (+)-N,N′-diallyltartardiamide using AIBN initiator under solvothermal conditions. A detailed characterization of this functionalized porous polymer is performed using N₂ sorption, solid state ¹³C CP MAS NMR, FT-IR and UV-Vis spectroscopy, HR-TEM, FE-SEM, TGA/DTA and CHN analysis. Photoluminescence study of the material is carried out to investigate the sensing behaviour of PDVTD-1 towards different nucleic acid bases. It is observed that at very low concentration of base (i.e. 10⁻⁶ to 10⁻⁷ M) PDVTD-1 can selectively sense cytosine, whereas in the concentration range 10⁻³ to 10⁻⁵ M adenine, thymine and uracil also shows quenching of its fluorescence intensity. Moreover, this N-rich porous polymer PDVTD-1 showed excellent CO₂ uptake capacity of 8.76 mmol g⁻¹ (38.54 wt%) at 273 K and 3 bar pressure with an initial isosteric heat of adsorption (Q_st) of 72 kJ mol⁻¹.

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Introduction

Nucleic acid base purines and pyrimidines are the major constituents of DNA and RNA of living organisms in nature. Owing to rapid cell division and intracellular activities, cancer cells have a huge quantity of these nucleic acids. In this context, designing materials for the selective detection of nucleic acids by sensing those bases¹ can be an important investigative tool to identify the neoplastic cells. Although, radiological scanning is an advanced technique employed frequently for their detection, selective biosensing of nucleic acid bases is an interesting field of research, which is devoid of radioactive hazards. Thus, nucleic acid probe based biosensing using a porous nanomaterial is a furthermore step in gene sequence mapping. Hence, genosensor mapping on whole human genome may open a new window in genotherapy.² The different donor–donor–acceptor functional groups present in the polymeric matrix can interact with the complementary acceptor–acceptor–donor array of purine and pyrimidine bases via intermolecular hydrogen bonding interaction³ and thus can show considerable shift in the photoluminescence spectra at higher excitation wavelength. Further, in contrast to their non-porous analogue, porous materials having high BET (Brunauer–Emmett–Teller) surface area and pore volume, which can allow improved adsorption and sensing property as the biomolecules can interact more with the chromophoric groups present on the surface.⁴

On the other hand, with rapid industrialization in last two decades the atmospheric CO₂ level is rising enormously, leading to an adverse effect in global environment in the forthcoming centuries. Due to advancement of human civilization, activities like deforestation, burning of fossil fuel and waste, industrial plants have polluted the air by emitting large amount of CO₂, affecting the natural eco-balance in the atmosphere. This seemingly large concentration of CO₂ is causing the ‘Earth's greenhouse’ effect, responsible for global warming. Hence, storage of CO₂ by means of physical adsorption can be an effective method to reduce the pollution. Now-a-days, physisorption at the porous material surface is very lucrative and promising due to their high surface area/porous texture, and N-rich basic framework sites, which could have stronger dipolar interaction with CO₂ molecules.⁵ However, a wide range of microporous materials like porous organic polymers (POPs),⁶ metal organic frameworks (MOFs),⁷ covalent organic frameworks (COFs),⁸ zeolitic imidazole frameworks (ZIFs),⁹ porous carbons,¹⁰ BCN graphene analogues¹¹ etc. have gained much attraction for large CO₂ storage capacity by physical adsorption in the recent time.

Polymers are materials with numerous applications in various fields such as optoelectronics, pharmacy, agriculture, medicinal chemistry due to their unique characteristics in terms of mechanical properties, stimuli responsiveness, good flexibility and high thermal stability. Due to the absence of periodicity in nanostructure, POPs have some drawbacks, like they do not exhibit any definite supramolecular pattern. However, still these polymers have attracted immense attention due to their ease of synthesis¹² compared to other classes of porous organic/organic–inorganic hybrid materials like covalent organic frameworks (COFs), covalent imine networks (CINs) and metal organic frameworks (MOFs). The porous polymer can be designed using single or multiple monomers to produce a 3D network structure via step growth and chain growth co-polymerization processes. Thus POPs with high specific BET surface area and bearing suitable active functional groups in the narrow pore wall can show fascinating applications like gas storage/fixation,¹³ sensing,¹⁴ base catalysis,¹⁵ Li-battery,¹⁶ and catalysis for the synthesis of fine chemicals etc.¹⁷ Typically, these POP materials have been synthesized by using different synthetic pathways, like radical polymerization,¹⁸ solvothermal synthesis,¹⁹ condensation²⁰ etc. and further incorporation of active metal ions in POPs can make them active heterogeneous catalysts for different chemical transformations under liquid phase conditions. To the best of our knowledge only very few reports are there in the literature regarding sensing of nucleic acid bases by porous organic polymers. Recently, Bhunia et al. have reported a porous organic polymer with high selectivity in sensing cytosine.^1c In our present manuscript we have focused on the synthesis of such high surface area porous organic polymer PDVTD-1, containing suitable functionality with the desired donor–donor–acceptor arrangement for sensing of above mentioned nucleotide bases. PDVTD-1 showed high selectivity in cytosine sensing at low concentration of nucleic acid base and good sensitivity for detection up to 0.5 μM concentration further, as this polymer possesses high N-content (surface basicity), microporosity and high surface area, which are crucial for carbon dioxide storage. We have studied its CO₂ uptake property at two different temperatures. PDVTD-1 showed high CO₂ uptakes, high isosteric heat of adsorption, which confirms strong binding, good reusability and reversibility of the adsorption process.

Experimental section

Materials

Divinylbenzene (DVB; M = 130.19 g mol⁻¹) and (+)-N,N-diallyltartardiamide (TD; M = 228 g mol⁻¹) used as monomers for the synthesis of PDVTD-1 were obtained from Sigma Aldrich, India. Azobisisobutyronitrile (AIBN) was received from SRL which was recrystallized using hot ethanol prior to use. The nucleotide bases, cytosine, adenine, thymine and uracil, employed for the biosensing studies were purchased from Sigma Aldrich, India. Other chemicals of analytical grade purchased from E-Mark, were used after purification.

Instrumentation

To record the N₂ adsorption/desorption isotherm Quantachrome Autosorb 1C was used at 77 K. Prior to gas adsorption, sample was degassed for 12 h at 393 K under high vacuum. NLDFT pore size distribution was calculated from the adsorption/desorption isotherm using the carbon/slit-cylindrical pore model. CO₂ adsorption/desorption isotherm was obtained by using a Bel Japan Inc. Belsorp-HP at 273 K and the degassing temperature for the porous polymer was set at 393 K for overnight. The FT-IR spectra of the samples were recorded using a Perkin-Elmer Spectrum 100 spectrophotometer. The UV-visible spectra of the polymers were recorded using UV 2401 PC with an integrating sphere attachment where BaSO₄ was used as the background standard. ¹³C CP MAS NMR spectrum of the sample was obtained on a Bruker Advance 500 MHz NMR spectrometer using a 4 nm MAS probe under static condition (spinning rate 5000 Hz, with side band suppression). High resolution transmission electron microscopy (HR TEM) images of the sample were obtained from JEOL 2010 TEM at 200 kV. To analyze the morphology and particle size of the samples JEOL JEM 6700 field emission scanning electron microscope (FE SEM) was used. Elemental analysis of the material was carried out using a Perkin Elmer 2400 Series II CHN analyzer. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) of the sample were performed in a TGA instrument thermal analyzer TA-SDT Q-600 under air flow. The emission spectra were recorded on a Varian Cary Eclipse luminescence spectrometer (excitation wavelength 265 nm, excitation and emission slit width both 5 nm).

Synthesis of microporous polymer (PDVTD-1)

The porous polymer PDVTD-1 has been synthesized through copolymerization of divinylbenzene and (+)-N,N′-diallyltartardiamide in the presence of a radical initiator AIBN under static solvothermal conditions. We have followed similar experimental conditions as that for the synthesis of our reported high surface area porous organic polymer PDVTA-1, where triallylamine was used as a monomer along with divinylbenzene.^6d In a typical synthesis of PDVTD-1, divinylbenzene (4 mmol, 0.521 g) and (+)-N,N′-diallyltartardiamide (1 mmol, 0.228 g) were taken in a cleaned 50 mL round bottom flask containing 10 mL acetone and the solution mixture was stirred for 15–20 min.

Then before addition of AIBN (0.020 g) to the mixture, the round bottom flask was purged with N₂ gas for 10 min to make inert atmosphere inside the flask. The whole system was stirred for around 2 h at room temperature (26 °C) to get the slurry. Then the resultant slurry was put in Teflon line stainless steel autoclave for 24 h at 120 °C temperature under static condition. The final white solid product was washed for 4–5 times with acetone and air-dried at room temperature. The synthetic pathway for the polymerization has been demonstrated in Scheme 1. To optimize maximum BET surface area, we have prepared two other polymers under similar reaction conditions with DVB to TD molar ratios of 9 : 1 and 7 : 3, and these are denoted as PDVTD-2 and PDVTD-3 respectively. Both the polymers showed lower surface area compared to PDVTD-1. A pure poly tartardimide (PTD) polymer was also synthesized without using any DVB for comparison, using similar experimental conditions. However, the aforesaid self-polymerized material showed very low surface area. Scheme 2 illustrates a possible mechanistic pathway for the formation of the micropores in PDVTD-1 polymeric network. The porous polymer PDVTD-1 with high BET surface area has been characterized thoroughly and employed for biomolecule sensing as well as for high CO₂ capture.

Scheme 1 Schematic representation for the synthesis of PDVTD-1 and its framework.

Scheme 2 Probable mechanism for formation of microporous polymer PDVTD-1.

Sensing experiment

For this experiment 15 mg of material was taken in 1 L deionised water and treated in an ultra-sonic bath for 10 minutes. Four different sets of aqueous solutions with varying concentrations (10, 50, 100, 500, 1000) μM were prepared for cytosine, adenine, thymine and uracil. To compare the binding affinity between different nucleotide bases and PDVTD-1, 1.5 mL of each of the purine and pyrimidine base solution was added to 1.5 mL of the aqueous suspension of PDVTD-1 and the quenching in the photoluminescence spectra was studied (excitation wavelength 265 nm, at 298 K). The experiment was repeated with further lower concentration of cytosine (0.5, 1, 2.5, 5) μM for which 2 mg L⁻¹ aqueous suspension of PDVTD-1 was used keeping all other experimental conditions unchanged.

Results and discussions

Spectroscopic analysis

The organic functionalities in the polymer framework of the porous polymer have been characterized by using Fourier transform infrared and UV-visible spectroscopy. The FTIR spectrum of the polymer PDVTD-1 and self polymerized PTD have been shown in Fig. 1 where both exhibits broad intense peak at around 3330–3425 cm⁻¹ due to the presence of secondary amine and hydroxyl group. Bands at 1128, 1060, 1545 and 1652 cm⁻¹ could be attributed to the C–N and C–O stretching, N–H bending vibration of amide and carbonyl stretching of the amide groups, respectively.²¹ The two peaks appearing at 2976 cm⁻¹ and 2920 cm⁻¹ correspond to the methylene C–H stretching vibration. The additional peaks at 1606 cm⁻¹ and 1450 cm⁻¹ indicates the aromatic C=C stretching vibration of the benzene ring of DVB part in case of PDVTD-1. The absence of these specific vibration bands represents no DVB in the self polymerized TD polymer as shown in Fig. 1b. For out-of plane aromatic C–H bending vibration, the peaks appear around at 700–800 cm⁻¹. Thus the IR spectrum confirms the incorporation of two starting materials in PDVTD-1 sample. Fig. 2a and b represent the UV-visible absorbance spectra of two materials PDVTD-1 and PTD, respectively. In Fig. 2a, the weak band at 218 nm and broad band from 250 nm to 292 nm could appear due to π– π* transition of the heteroatom in presence of aromatic systems.²² On the other hand, only one band at 210 nm observed for the PTD material bearing no aromatic moiety (Fig. 2b). So the different functional moieties present in PDVTD-1 could be responsible for the multiple bands in its UV-visible spectrum.

Fig. 1 FT IR spectra of PDVTD-1 (a) and PTD (b).

Fig. 2 UV-Vis spectra of PDVTD-1 (a) and PTD (b) materials.

Solid state ¹³C CP MAS NMR

Further, the solid state ¹³C CP MAS NMR analysis technique confirms the presence of desired organic functional groups in the polymer as well as the chemical environment of C atom corresponding to the different functionalities. The ¹³C CP MAS NMR spectrum shows strong signals at 29.2, 41.5 ppm and the chemical shift further downfield at 127.7, 137.1, 146.2 ppm due to aliphatic and aromatic C atom of the polymeric network respectively, which is shown in Fig. 3. The two signals at 73 and 80 ppm corresponds to the C atoms bearing the hydroxyl group, whereas, the weak signal at 112.2 represents trace amount of unreacted olefin. As shown in Fig. 3, the very weak signal at 15.6 ppm is appeared due to the presence of aliphatic sp³ C atom in the polymeric framework. However, the solid state ¹³C CP MAS NMR experiment demonstrates the complete formation of the desired polymer.

Fig. 3 ¹³C CP MAS NMR spectrum of PDVTD-1.

Porosity and surface area measurement

The porosity and BET surface area of PDVTD-1 has been estimated from N₂ sorption analysis at 77 K, where N₂ is used as the adsorbate molecule. The N₂ adsorption/desorption isotherm of PDVTD-1 (Fig. 4a) is mainly of type I pattern, characteristic of microporous material. The adsorption isotherm with large uptake at relatively low pressure region (0–0.1 bar) represents good microporosity in the polymer moiety. The BET (Brunauer–Emmett–Teller) surface area and pore volume of PDVTD-1 is 621 m² g⁻¹ and 0.287 ccg⁻¹, respectively. The non local density functional theory (NLDFT) using N₂ at 77 K on a carbon/slit-cylindrical pore model shows the pore diameter of 1.2 nm. Possible deviation in pore diameters is ±0.2 nm. According to de Boer statistical thickness (t-plot) the surface area of PDVTD-1 sample due to microporosity and mesoporosity has been estimated which are 585 m² g⁻¹ and 36 m² g⁻¹ respectively. This result suggests that PDVTD-1 is highly microporous in nature. Fig. 4b shows the N₂ adsorption/desorption isotherm of the polymers PDVTD-2 and PDVTD-3, which correspond to surface areas 525 and 290 m² g⁻¹, respectively. The N₂ sorption analysis of the polymer PTD reveals very low surface area (1.23 m² g⁻¹) and a pore volume of 0.0004 ccg⁻¹ (isotherm not shown).

Fig. 4 N₂ adsorption/desorption isotherm of PDVTD-1 (above) and PDVTD-2 (a), PDVTD-3 (b) (below). Adsorption and desorption points are marked by filled and empty symbols, respectively. The pore size distribution pattern employing the nonlocal density functional theory (NLDFT) is shown in the inset for PDVTD-1 sample.

The HR-TEM image showed micropores of around 1.12 nm diameter arranged all over the polymer framework as shown in Fig. 5. On the other hand the FE-SEM image revealed the structural morphology of the PDVTD-1 material as shown in Fig. 6. Aggregation of tiny spherical particles of 28–32 nm diameter gives a self-assembled globular morphology with dimension about 150–170 nm. Due to high BET surface area and presence of the secondary amine groups and hydroxyl group in a specific arrangement, the polymer PDVTD-1 reveals interesting sensing behaviour and high CO₂ uptake, which are discussed in the following sessions.

Fig. 5 HR-TEM images of the porous polymer PDVTD-1.

Fig. 6 FE-SEM image of the porous polymer PDVTD-1.

Elemental analysis and thermal stability

CHN analysis data reveals that the presence of carbon, hydrogen and nitrogen in our porous polymer where C = 79.79%, H = 7.37% and N = 3.60% whereas theoretical calculation from the stoichiometry of starting materials gives the C, H, N and O content to be 79.9%, 7.45%, 3.72% and 8.52% respectively. To investigate the thermal stability of porous polymer PDVTD-1, thermogravimetric analysis has been studied under air flow at 10 °C temperature ramp. Major weight loss of the material from 401 °C to 470 °C is observed from the TGA–DTA plot which is shown in Fig. 7. It suggested that the porous organic polymer PDVTD-1 has considerably high thermal stability. The weight loss of the material could be attributed due to C–C and C–N bond cleavage.

Fig. 7 TGA (a) and DTA (b) plots for PDVTD-1.

Surface basicity analysis by acid–base titration

Acid–base titration of PDVTD-1 using phenolphthalein as indicator gave a surface basicity of 1.736 mmol g⁻¹, which is sufficiently high. Using (M/100) standardise NaOH and (M/100) oxalic acid solution, the basicity of PDVTD-1 has been measured. High surface basicity of PDVTD-1 has motivated us to explore its application in CO₂ storage.

Sensing behaviour

Polymers with high surface area containing specific donor–donor–acceptor functional group arrangement can interacts with nucleotide bases cytosine, adenine, thymine, uracil and can show certain quenching or enhancement in the photoluminescence signal of the material. Fig. 8 represents the effect of addition of various concentrations (0, 10, 50, 100, 500, 1000 μM) of these nucleotide bases on the fluorescence spectrum of the aqueous suspension (15 mg L⁻¹) of the polymer PDVTD-1. Upon excitation at 265 nm at 298 K the material showed an emission maximum at 326 nm, which gradually decreases when the nucleotide bases with increasing concentrations are added. It was observed that for lower concentration of bases (10 μM), only cytosine and adenine shows visible fluorescence quenching, whereas for thymine and uracil the emission spectra of PDVTD-1 remains unchanged. Fig. 9 shows a comparative study of the fluorescence quenching efficiency, (I₀ − I)/I₀, of PDVTD-1 with varying nucleotide bases of 10 μM concentration. Since the material is found to be selective for detection of cytosine in lower concentration range, we have continued the experiment with cytosine for further lower concentrations. The effect of addition of 0.5, 1, 2.5 and 5 μM cytosine solution to PDVTD-1 aqueous suspension on the fluorescence emission is shown in Fig. 10a.

Fig. 8 Emission spectra of PDVTD-1 (15 mg L⁻¹, aqueous suspension) with gradual addition of nucleotide bases (a) cytosine, (b) adenine, (c) thymine and (d) uracil. Excitation wavelength = 265 nm. [Nucleotide bases] = (0, 10, 50, 100, 500, 1000) μM.

Fig. 9 Fluorescence quenching efficiency, (I₀ − I)/I₀, of PDVTD-1 (15 mg L⁻¹, aqueous suspension) for different nucleotide bases. I and I₀ are the maximum fluorescence intensity of PDVTD-1 with and without nucleotide bases, respectively. Excitation wavelength = 265 nm. [Nucleotide bases] = 10 μM.

Fig. 10 (a) Emission spectra of PDVTD-1 (2 mg L⁻¹, aqueous suspension) with gradual addition of cytosine. Excitation wavelength = 265 nm. [Cytosine] = (0, 0.5, 1, 2.5, 5) μM. The I₀/I vs. concentration plot (Stern–Volmer plot) shown in inset, where I and I₀ are the maximum fluorescence intensity of PDVTD-1 with and without cytosine. The quenching constant (K_SV) was obtained to be 1.5 × 10⁶ M⁻¹. (b) Interaction of donor–donor–acceptor sites of PDVTD-1 with the corresponding sites of cytosine.

The figure in the inset illustrates a relation between the fluorescence quenching and concentration of cytosine. From this the quenching constant (K_SV) of PDVTD-1 is obtained as 1.5 × 10⁶ M⁻¹ using the Stern–Volmer equation I₀/I = 1 + K_SV[Q]. A better correlation between the counter-sites, i.e. the donor–donor–acceptor sites of PDVTD-1 and acceptor–acceptor–donor array of cytosine, suitable distance between the binding sites and strong H-binding affinity may be responsible for the higher sensing selectivity of PDVTD-1 towards cytosine.²³ A probable interaction of PDVTD-1 with cytosine is shown in Fig. 10b. The correlation between the PL intensity and concentration of the nucleic acid bases are shown in Fig. 11, which displays almost linear dependence. This result suggested no self-quenching of the fluorescent nucleic acid bases is occurring.

Fig. 11 The linear correlation plots between florescence intensity and the concentration of the nucleic acid bases.

Carbon dioxide uptake

Fig. 12 demonstrates the CO₂ adsorption/desorption isotherms for PDVTD-1 sample at two different temperatures 273 and 298 K up to 3 bar pressure, which shows that with increasing pressure, uptake of CO₂ increases. Though in high pressure region the rate of adsorption gradually decreases, but the still increasing nature of the isotherms indicate that PDVTD-1 can uptake some more CO₂ if the pressure is further increased. The total amount of CO₂ uptake by PDVTD-1 polymer are 8.76 mmol g⁻¹ (38.54 wt%) and 1.06 mmol g⁻¹ (4.66 wt%) at 273 K and 298 K temperature, respectively under 3 bar pressure. As physical adsorption is accompanied by a decrease in free energy and entropy of the adsorption system and thus it is an exothermic process. So, with increase in temperature the reverse process i.e. desorption becomes more facile. So, the uptake amount is high at 273 K compared to that at 298 K.²⁴

Fig. 12 CO₂ uptake profiles of PDVTD-1 at two different temperatures 273 and 298 K.

The temperature dependence of the adsorption/desorption equilibrium pressure can be described by Clausious–Clapeyron equation, which gives the isosteric heat of adsorption for PDVTD-1 (Fig. 13) of 72.8 kJ mol⁻¹. The heat of adsorption (Q_st) is a function of CO₂ concentration and the exceptionally high initial Q_st value represents strong interaction between adsorbate and adsorbent species. But the desorption isotherm indicates that the material can be easily recovered without the aid of any external energy.^10b This polymeric framework displays good amount of CO₂ uptake not only for high BET surface area, also due to the presence of basic groups and microporosity in the polymer matrix. In order to check reusability of the material, further CO₂ adsorption study has been carried out for four times. After completion of last cycle there is a very small decrease in amount of CO₂ uptake (38.54 to 37.05 wt%), suggesting PDVTD-1 material has very good potential for efficient adsorption for several adsorption cycles (Fig. 14).

Fig. 13 Isosteric heat of adsorption (Q_st) of PDVTD-1 as a function of the amount of CO₂ adsorbed.

Fig. 14 Reusability test of PDVTD-1 for CO₂ adsorption.

Conclusions

In summary, we have synthesized a new highly porous co-polymer (PDVTD-1) of divinylbenzene and (+)-N,N′-diallyltartardiamide using radical polymerization pathway. The copolymer has been characterized using ¹³C CP MAS NMR, FT-IR and UV-Vis spectroscopy, HR-TEM, FE-SEM, N₂ sorption analysis, TGA/DTA and CHN analyses. The sensing property of the polymer has been thoroughly investigated for various nucleotide bases using photoluminescence spectroscopy. The material can detect cytosine at 0.5 μM concentration, whereas it can sense adenine, thymine and uracil over 10 μM concentrations. Particular matching in the arrangement of the donor–donor–acceptor sites may be responsible for such selective sensing behaviour. Further, this porous polymer showed an uptake of 8.76 mmol g⁻¹ CO₂ at 273 K under 3 bar pressure, which could open new avenues for this N-rich porous polymer in environmental research.

Acknowledgements

PB and RG thank CSIR, New Delhi for their respective junior and senior research fellowships. AB wishes to thank DST, New Delhi for instrumental facilities through DST Unit on Nanoscience, DST-SERB and DST-UKIERI project grants.

Notes and references

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