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Cite this: Chem. Commun., 2014, 50, 11311
DNA sequence-dependent photoluminescence enhancement in a cationic conjugated polyelectrolyte†
Received 6th May 2014, Accepted 26th July 2014
Zhongwei Liu,ab Hsing-Lin Wang*c and Mircea Cotlet*ab
DOI: 10.1039/c4cc03417a www.rsc.org/chemcomm
DNA sequence-dependent photoluminescence enhancement is found for a cationic polyelectrolyte complexed with single stranded DNA and described as a result of an interplay between electrostatic attraction and the p–p stacking between the polyelectrolyte’s backbone and DNA’s bases.
Cationic conjugated polyelectrolytes (CPs) have positively charged side groups rendering water solubility and the ability to bind negatively charged biomolecules like DNA and proteins. Cationic CPs have been proposed for DNA sensing owing to their chain conformation and aggregation state changes induced upon binding with DNA, changes that can produce colorimetric/fluorimetric signals that in turn can provide a transduction mechanism for DNA detection.1–6 The bulk of the studies involving cationic CPs and DNA have focused on the development of DNA sensing platforms, whether with increased detection sensitivity or with sequence specificity.3,7,8 Recently, several studies have attempted to address the mechanism of interaction and the interplay of various intermolecular interactions between DNA and cationic CPs.5,9–12 Here we report DNA sequence-dependent photoluminescence (PL) enhancement for a cationic CP, a poly{2,5-bis [3-(N,N,Ntriethylammonium)-1-oxapropyl]-1,4-phenylene vinylene}-dibromide (C-PPV, Fig. 1a, MW = 15 kDa),13 when it is complexed with single stranded (ss) DNA, with the PL enhanced as high as seven fold. C-PPV alone absorbs at 442 nm and emits at 530 nm, with both UV-vis absorption and PL spectra broad and structureless (Fig. 2a–j, black curves). Five types of 25 mer ssDNA oligomers were used in the present study, homo oligomeric DNAs, ssDNAd(A)25, ssDNAd(C)25, ssDNAd(G)25 and ssDNAd(T)25, and a random sequence, ssDNA(R)25, 5 0 -ATT a
Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY 11973, USA. E-mail: [email protected]; Fax: +1 631-344-7765; Tel: +1 631-344-7778 b Materials Science and Engineering Department, Stony Brook University, Stony Brook, NY 11794, USA c Chemistry Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA. E-mail: [email protected] † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c4cc03417a
This journal is © The Royal Society of Chemistry 2014
Fig. 1
Chemical structures of cationic C-PPV (a) and non-ionic N-PPV (b).
GTC TGT GTC TGG TGT GCG TCT G-3 0 . C-PPV was mixed with each oligomer at various molar ratios in 10 mM phosphate buffered saline (PBS, pH 7.4) and the UV-Vis absorption and PL spectra of the resulting complexes are shown in Fig. 2. Complexation of C-PPV with DNA induces red spectral shifts in both UV-vis absorption and PL spectra (Fig. 2) and the PL spectrum of each complex becomes vibronically structured. Table 1 lists the spectral shifts for UV-vis absorption (Dlabs) and PL (DlPL) spectra observed from C-PPV alone to C-PPV complexed with ssDNA of various sequences. Complexation of C-PPV with ssDNA results in PL enhancement for all sequences, except ssDNAd(G)25 (Table 1, QYcomplex/QYC-PPV and Fig. S1, ESI†). The PL enhancement is sequence dependent, strongest for C-PPV–ssDNAd(T)25 (7.3 fold), weakest for C-PPV–ssDNAd(C)25. It is noteworthy that the UV-vis absorption spectra vs. added DNA feature pseudo-isosbestic points for all five complexes (Fig. 2), suggesting a transition of the C-PPV between two states. Pseudoisosbestic points were also observed in the PL spectra of these complexes with added DNA, except for C-PPV–ssDNA(G)25 where a gradual red shift occurred (Fig. 2j). This together with the observation of vibronically structured and enhanced PL emission from C-PPV–DNA complexes suggest that complexation of C-PPV with DNA involves a polymer chain conformational change from a quenched state to a bright emitting state accompanied by an extended chain conformation, rather than polymer chain
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Fig. 2 Absorption and PL spectra in 10 mM PBS buffer (pH 7.4) for C-PPV (black curves in each panel) and C-PPV complexed with ssDNAd(R)25 (a and b), ssDNAd(A)25 (c and d), ssDNAd(C)25 (e and f), ssDNAd(T)25 (g and h) and ssDNAd(G)25 (i and j) at various C-PPV : DNA molar ratios, from 10 : 1 up to 1 : 3, with C-PPV at 0.3 mM. Arrows indicate increase in DNA concentration.
Table 1 Spectroscopic parameters derived from UV-vis absorption and PL data from Fig. 2 and Fig. S3 and S4, ESI; absorption shift, Dlabs; PL shift, DlPL; PL enhancement, QYcomplex/QYC-PPV (see the ESI for details on calculation). Values were calculated for data corresponding to C-PPV only (0.3 mM), C-PPV–ssDNA and N-PPV–ssDNA at a 1 : 3 molar ratio
Complex
Dlabs/nm
DlPL/nm
QYCOMPLEX/QYC-PPV
C-PPV–ssDNAd(C)25 C-PPV–ssDNAd(T)25 C-PPV–ssDNAd(A)25 C-PPV–ssDNAd(G)25 C-PPV–ssDNAd(R)25 C-PPV in DMSO N-PPV–ssDNAd(A)25 N-PPV–ssDNAd(C)25 N-PPV–ssDNAd(T)25 N-PPV–ssDNAd(G)25
58 43 38 18 48 44 132 0.0 0.0 0.0
40 22 21 25 30 28 0.0 0.0 0.0 0.0
1.97 7.33 3.77 0.83 3.87 6.30 0.81 0.00 0.00 0.50
aggregation state change.14,15 CP aggregation results usually in quenched, spectrally red shifted, and rather broad PL emission.14 C-PPV complexed with ordinary anionic polyelectrolytes like poly(vinyl sulphonic acid) (PVSa) results in the formation of self-quenched aggregates with broad, structureless absorption and PL spectra and with the PL severely quenched.8 For C-PPV– ssDNAd(G)25, PL quenching with added DNA is most probably a result of photo-induced electron transfer between guanine (G) and the C-PPV’s phenylene vinylene backbone.16,17 Circular dichroism (CD) spectroscopy of C-PPV–ssDNA complexes (Fig. S2, ESI†) shows evidence of strong interaction
11312 | Chem. Commun., 2014, 50, 11311--11313
between the two moieties for all sequences, less for ssDNAd(G)25. DNA alone (red curves in Fig. S2b–f, ESI†) shows negative and positive bands in the 200–300 nm region, and they are a result of the right-hand stacking of DNA bases. C-PPV alone is achiral (Fig. S2a, ESI†), but attains chiroptical activity in the visible region (400–600 nm) upon complexation with ssDNA, (Fig. S2b–f, red curves, ESI†), provided an excitonic system is induced in the C-PPV–ssDNA complex. CD spectroscopy of C-PPV–ssDNA complexes shows strong perturbation of the bands associated with DNA (200–300 nm), less for ssDNAd(G)25 (Fig. S2e, ESI†), that is, for all complexes exhibiting PL enhancement (Table 1 and Fig. S1, ESI†). This suggests strong interactions occurring between the hydrophobic parts of the two moieties (see below), and disturbing the right-hand stacking of the ssDNA and its chirality. This results in weak DNA-induced chiroptical signals in the visible region for C-PPV–DNA complexes when compared to those reported for other cationic CPs complexed with DNA.2,9,18 The positively charged side groups of C-PPV are vital for prompting the interaction between C-PPV and DNA that leads to PL enhancement.2,19 Indeed, a non-ionic PPV, poly(2,5-bis (diethylamine tetraethylene glycol)phenylene vinylene) (N-PPV, MW 32 kDa), with an identical backbone to C-PPV, shows neither spectral changes nor shape changing in absorption and PL spectra when mixed with ssDNAd(C)25, ssDNAd(T)25, ssDNAd(R)25 and ssDNAd(G)25. N-PPV mixed with ssDNAd(G)25 exhibits dramatic PL quenching (50%), reconfirming the possibility of photoinduced electron transfer occurring between the PPV’s backbone and guanine. N-PPV mixed with ssDNAd(A)25 shows formation of self-quenched aggregates by the occurrence of a red shifted shoulder (590 nm) in the absorption spectrum (Fig. S3, ESI†) and quenched PL (Table 1). In conjugated polymers, conformational disorder in the polymer backbone can exist in the form of chemical defects, backbone torsion or coiling20–22 and this can affect the p-system conjugation length and the resulting spectroscopic properties. For C-PPV, the hydrophobic backbone self-coils in water to reduce polymer–solvent interaction, breaking the p-system conjugation length and promoting self-quenching.23 Spectroscopic changes similar to those observed for C-PPV complexed with DNA, including PL enhancement, could be reproduced when C-PPV was dissolved in a mixed water–dimethyl sulfoxide (DMSO) solution, with DMSO being a better solvent for C-PPV than water (see Table 1 and Fig. S4, ESI†). Thus, this confirms the hypothesis of a polymer chain conformational change for C-PPV when complexed with DNA. Next to electrostatic attraction, leading to a polymer chain conformational change in the case of C-PPV complexed with DNA, hydrophobic interactions also define the resulting spectroscopic properties of the complex, as suggested for other heterocyclic aromatic conjugated polyelectrolytes interacting with DNA.10,18 Complexation between C-PPV and DNA strongly perturbs the right-hand stacking of DNA bases as shown by CD spectra (Fig. S2, ESI†), suggesting strong hydrophobic interactions between the C-PPV’s backbone and DNA bases. For C-PPV, hydrophobic interactions occur most probably as p–p stacking between the C-PPV’s phenylene vinylene backbone and DNA bases. p–p stacking increases the p-conjugation system, here
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next to C-PPV’s polymer chain unfolding induced by the electrostatic attraction with the DNA’s phosphate backbone. As such, both interactions will contribute to the observed red shift of the absorption spectrum of the polymer (Fig. 2 and Table 1, Dlabs). Assuming that the electrostatic interaction between C-PPV’s charged side groups and the DNA’s phosphate backbone is similar among different sequences, the contribution of this particular interaction to the overall red shift of the absorption spectrum should be similar among different DNA sequences complexed with C-PPV. As such, the magnitude of the observed red shift in absorption for a given C-PPV–DNA complex (Table 1, Dlabs) quantifies the strength of the p–p stacking between the C-PPV’s backbone and DNA bases. The largest spectral shift in absorption, e.g. strongest p–p stacking, occurs for C-PPV–ssDNAd(C)25 (58 nm), followed by ssDNAd(T)25 (43 nm), ssDNAd(A)25 (38 nm) and ssDNAd(G)25 (18 nm). From these absorption spectral shifts we hypothesize that the C-PPV’s phenylene vinylene backbone, a six member aromatic ring, interacts stronger with single (six-member) ring pyrimidines (C,T bases) than with double ring pyrimidines (C,G bases). These findings are in contrast to those previously reported for cationic polythiophenes interacting with DNA where stronger stacking was observed with purines.11 This is not controversial because a five-membered aromatic ring like thiophene might prefer stacking with a homologous ring that is present only in purines. C-PPV–ssDNAd(G)25 behaves differently than all other complexes, featuring small spectral shift in absorption and quenched PL. G oligonucleotides are known to form secondary structures24 which in this case might prevent p–p stacking with C-PPV. As such, in a C-PPV–ssDNAd(G)25 complex, interaction between C-PPV and DNA will be purely electrostatic attraction, resulting in turn in the smallest observed red shift in absorption (Fig. 2i and Table 1). We demonstrated DNA sequence specific PL enhancement of a conjugated polyelectrolyte complexed with homo and hetero oligonucleotides and discussed it as resulting from an interplay between electrostatic and hydrophobic interactions. Electrostatic attraction between polymer’’s charged side groups is essential in promoting hydrophobic interactions responsible for the observed PL enhancement, here in the form of p–p stacking between the polymer backbone and the DNA bases. Further engineering of such conjugated polyelectrolytes might provide complexes with tunable PL and chiroptical properties that can be useful in the development of biosensors with sequence specific recognition. Research was carried out at the Center for Functional Nanomaterials, Brookhaven National Laboratory supported by
This journal is © The Royal Society of Chemistry 2014
Communication
the U.S. Department of Energy, Office of Basic Energy Sciences, Contract No. DE-AC02-98CH10886 (M.C., Z.L.) and by the Biomaterials Program of the Division of Materials Science and Engineering, Office of Basic Energy Sciences (H.L.W.). We thank Fang Lu from Brookhaven Laboratory and Prahlad K. Routh from Stony Brook University for help with some of the experiments reported herein.
Notes and references 1 C. L. Zhu, L. B. Liu, Q. Yang, F. T. Lv and S. Wang, Chem. Rev., 2012, 112, 4687. ´, 2 H.-A. Ho, M. Boissinot, M. G. Bergeron, G. Corbeil, K. Dore D. Boudreau and M. Leclerc, Angew. Chem., Int. Ed., 2002, 41, 1548. 3 H. A. Ho, M. Bera-Aberem and M. Leclerc, Chem. – Eur. J., 2005, 11, 1718. 4 J. Sun, Y. Lu, L. Wang, D. Cheng, Y. Sun and X. Zeng, Polym. Chem., 2013, 4, 4045. 5 K. Peter, R. Nilsson and O. Inganas, Nat. Mater., 2003, 2, 419. 6 H. Song, B. Sun, K.-J. Gu, Y. Yang, Y. Zhang and Q.-D. Shen, J. Appl. Polym. Sci., 2009, 114, 1278. 7 B. Liu and G. C. Bazan, Chem. Mater., 2004, 16, 4467. 8 Z. Liu, H.-L. Wang and M. Cotlet, Chem. Mater., 2014, 26, 2900. 9 J. Rubio-Magnieto, A. Thomas, S. Richeter, A. Mehdi, P. Dubois, R. Lazzaroni, S. Clement and M. Surin, Chem. Commun., 2013, 49, 5483. 10 C. Chunyan, A. Chworos, Z. Jinping, A. Mikhailovsky and G. C. Bazan, Adv. Funct. Mater., 2008, 18, 3606. `re, E. W. Meijer and 11 M. Surin, P. G. A. Janssen, R. Lazzaroni, P. Lecle A. P. H. J. Schenning, Adv. Mater., 2009, 21, 1126. 12 F. Brustolin, M. Surin, V. Lemaur, G. Romanazzi, Q. Sun, J. Cornil, R. Lazzaroni, N. Sommerdijk, P. Leclere and E. W. Meijer, Bull. Chem. Soc. Jpn., 2007, 80, 1703. 13 Y. Gao, C.-C. Wang, L. Wang and H.-L. Wang, Langmuir, 2007, 23, 7760. 14 J. W. Hong, W. L. Hemme, G. E. Keller, M. T. Rinke and G. C. Bazan, Adv. Mater., 2006, 18, 878. 15 C.-C. Wang, Y. Gao, A. P. Shreve, C. Zhong, L. Wang, K. Mudalige, H.-L. Wang and M. Cotlet, J. Phys. Chem. B, 2009, 113, 16110. 16 S. Kurata, T. Kanagawa, K. Yamada, M. Torimura, T. Yokomaku, Y. Kamagata and R. Kurane, Nucleic Acids Res., 2001, 29, e34. 17 M. L. Davies, P. Douglas, H. D. Burrows, B. Martincigh, M. D. Miguel, U. Scherf, R. Mallavia and A. Douglas, J. Phys. Chem. B, 2014, 118, 460. 18 M. Surin, P. G. A. Janssen, R. Lazzaroni, P. Leclere, E. W. Meijer and A. Schenning, Adv. Mater., 2009, 21, 1126. 19 J. Rubio-Magnieto, A. Thomas, S. Richeter, A. Mehdi, P. Dubois, R. Lazzaroni, S. Clement and M. Surin, Chem. Commun., 2013, 49, 5483. 20 O. Lhost and J. L. Bredas, J. Chem. Phys., 1992, 96, 5279. 21 P. F. Barbara, A. J. Gesquiere, S. J. Park and Y. J. Lee, Acc. Chem. Res., 2005, 38, 602. 22 G. Rossi, R. R. Chance and R. Silbey, J. Chem. Phys., 1989, 90, 7594. 23 Z. Xu, H. Tsai, H.-L. Wang and M. Cotlet, J. Phys. Chem. B, 2010, 114, 11746. 24 A. I. Karsisiotis, N. M. Hessari, E. Novellino, G. P. Spada, A. Randazzo and M. W. da Silva, Angew. Chem., Int. Ed., 2011, 50, 10645.
Chem. Commun., 2014, 50, 11311--11313 | 11313
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Cite this: Chem. Commun., 2014, 50, 11311
DNA sequence-dependent photoluminescence enhancement in a cationic conjugated polyelectrolyte†
Received 6th May 2014, Accepted 26th July 2014
Zhongwei Liu,ab Hsing-Lin Wang*c and Mircea Cotlet*ab
DOI: 10.1039/c4cc03417a www.rsc.org/chemcomm
DNA sequence-dependent photoluminescence enhancement is found for a cationic polyelectrolyte complexed with single stranded DNA and described as a result of an interplay between electrostatic attraction and the p–p stacking between the polyelectrolyte’s backbone and DNA’s bases.
Cationic conjugated polyelectrolytes (CPs) have positively charged side groups rendering water solubility and the ability to bind negatively charged biomolecules like DNA and proteins. Cationic CPs have been proposed for DNA sensing owing to their chain conformation and aggregation state changes induced upon binding with DNA, changes that can produce colorimetric/fluorimetric signals that in turn can provide a transduction mechanism for DNA detection.1–6 The bulk of the studies involving cationic CPs and DNA have focused on the development of DNA sensing platforms, whether with increased detection sensitivity or with sequence specificity.3,7,8 Recently, several studies have attempted to address the mechanism of interaction and the interplay of various intermolecular interactions between DNA and cationic CPs.5,9–12 Here we report DNA sequence-dependent photoluminescence (PL) enhancement for a cationic CP, a poly{2,5-bis [3-(N,N,Ntriethylammonium)-1-oxapropyl]-1,4-phenylene vinylene}-dibromide (C-PPV, Fig. 1a, MW = 15 kDa),13 when it is complexed with single stranded (ss) DNA, with the PL enhanced as high as seven fold. C-PPV alone absorbs at 442 nm and emits at 530 nm, with both UV-vis absorption and PL spectra broad and structureless (Fig. 2a–j, black curves). Five types of 25 mer ssDNA oligomers were used in the present study, homo oligomeric DNAs, ssDNAd(A)25, ssDNAd(C)25, ssDNAd(G)25 and ssDNAd(T)25, and a random sequence, ssDNA(R)25, 5 0 -ATT a
Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY 11973, USA. E-mail: [email protected]; Fax: +1 631-344-7765; Tel: +1 631-344-7778 b Materials Science and Engineering Department, Stony Brook University, Stony Brook, NY 11794, USA c Chemistry Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA. E-mail: [email protected] † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c4cc03417a
This journal is © The Royal Society of Chemistry 2014
Fig. 1
Chemical structures of cationic C-PPV (a) and non-ionic N-PPV (b).
GTC TGT GTC TGG TGT GCG TCT G-3 0 . C-PPV was mixed with each oligomer at various molar ratios in 10 mM phosphate buffered saline (PBS, pH 7.4) and the UV-Vis absorption and PL spectra of the resulting complexes are shown in Fig. 2. Complexation of C-PPV with DNA induces red spectral shifts in both UV-vis absorption and PL spectra (Fig. 2) and the PL spectrum of each complex becomes vibronically structured. Table 1 lists the spectral shifts for UV-vis absorption (Dlabs) and PL (DlPL) spectra observed from C-PPV alone to C-PPV complexed with ssDNA of various sequences. Complexation of C-PPV with ssDNA results in PL enhancement for all sequences, except ssDNAd(G)25 (Table 1, QYcomplex/QYC-PPV and Fig. S1, ESI†). The PL enhancement is sequence dependent, strongest for C-PPV–ssDNAd(T)25 (7.3 fold), weakest for C-PPV–ssDNAd(C)25. It is noteworthy that the UV-vis absorption spectra vs. added DNA feature pseudo-isosbestic points for all five complexes (Fig. 2), suggesting a transition of the C-PPV between two states. Pseudoisosbestic points were also observed in the PL spectra of these complexes with added DNA, except for C-PPV–ssDNA(G)25 where a gradual red shift occurred (Fig. 2j). This together with the observation of vibronically structured and enhanced PL emission from C-PPV–DNA complexes suggest that complexation of C-PPV with DNA involves a polymer chain conformational change from a quenched state to a bright emitting state accompanied by an extended chain conformation, rather than polymer chain
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Fig. 2 Absorption and PL spectra in 10 mM PBS buffer (pH 7.4) for C-PPV (black curves in each panel) and C-PPV complexed with ssDNAd(R)25 (a and b), ssDNAd(A)25 (c and d), ssDNAd(C)25 (e and f), ssDNAd(T)25 (g and h) and ssDNAd(G)25 (i and j) at various C-PPV : DNA molar ratios, from 10 : 1 up to 1 : 3, with C-PPV at 0.3 mM. Arrows indicate increase in DNA concentration.
Table 1 Spectroscopic parameters derived from UV-vis absorption and PL data from Fig. 2 and Fig. S3 and S4, ESI; absorption shift, Dlabs; PL shift, DlPL; PL enhancement, QYcomplex/QYC-PPV (see the ESI for details on calculation). Values were calculated for data corresponding to C-PPV only (0.3 mM), C-PPV–ssDNA and N-PPV–ssDNA at a 1 : 3 molar ratio
Complex
Dlabs/nm
DlPL/nm
QYCOMPLEX/QYC-PPV
C-PPV–ssDNAd(C)25 C-PPV–ssDNAd(T)25 C-PPV–ssDNAd(A)25 C-PPV–ssDNAd(G)25 C-PPV–ssDNAd(R)25 C-PPV in DMSO N-PPV–ssDNAd(A)25 N-PPV–ssDNAd(C)25 N-PPV–ssDNAd(T)25 N-PPV–ssDNAd(G)25
58 43 38 18 48 44 132 0.0 0.0 0.0
40 22 21 25 30 28 0.0 0.0 0.0 0.0
1.97 7.33 3.77 0.83 3.87 6.30 0.81 0.00 0.00 0.50
aggregation state change.14,15 CP aggregation results usually in quenched, spectrally red shifted, and rather broad PL emission.14 C-PPV complexed with ordinary anionic polyelectrolytes like poly(vinyl sulphonic acid) (PVSa) results in the formation of self-quenched aggregates with broad, structureless absorption and PL spectra and with the PL severely quenched.8 For C-PPV– ssDNAd(G)25, PL quenching with added DNA is most probably a result of photo-induced electron transfer between guanine (G) and the C-PPV’s phenylene vinylene backbone.16,17 Circular dichroism (CD) spectroscopy of C-PPV–ssDNA complexes (Fig. S2, ESI†) shows evidence of strong interaction
11312 | Chem. Commun., 2014, 50, 11311--11313
between the two moieties for all sequences, less for ssDNAd(G)25. DNA alone (red curves in Fig. S2b–f, ESI†) shows negative and positive bands in the 200–300 nm region, and they are a result of the right-hand stacking of DNA bases. C-PPV alone is achiral (Fig. S2a, ESI†), but attains chiroptical activity in the visible region (400–600 nm) upon complexation with ssDNA, (Fig. S2b–f, red curves, ESI†), provided an excitonic system is induced in the C-PPV–ssDNA complex. CD spectroscopy of C-PPV–ssDNA complexes shows strong perturbation of the bands associated with DNA (200–300 nm), less for ssDNAd(G)25 (Fig. S2e, ESI†), that is, for all complexes exhibiting PL enhancement (Table 1 and Fig. S1, ESI†). This suggests strong interactions occurring between the hydrophobic parts of the two moieties (see below), and disturbing the right-hand stacking of the ssDNA and its chirality. This results in weak DNA-induced chiroptical signals in the visible region for C-PPV–DNA complexes when compared to those reported for other cationic CPs complexed with DNA.2,9,18 The positively charged side groups of C-PPV are vital for prompting the interaction between C-PPV and DNA that leads to PL enhancement.2,19 Indeed, a non-ionic PPV, poly(2,5-bis (diethylamine tetraethylene glycol)phenylene vinylene) (N-PPV, MW 32 kDa), with an identical backbone to C-PPV, shows neither spectral changes nor shape changing in absorption and PL spectra when mixed with ssDNAd(C)25, ssDNAd(T)25, ssDNAd(R)25 and ssDNAd(G)25. N-PPV mixed with ssDNAd(G)25 exhibits dramatic PL quenching (50%), reconfirming the possibility of photoinduced electron transfer occurring between the PPV’s backbone and guanine. N-PPV mixed with ssDNAd(A)25 shows formation of self-quenched aggregates by the occurrence of a red shifted shoulder (590 nm) in the absorption spectrum (Fig. S3, ESI†) and quenched PL (Table 1). In conjugated polymers, conformational disorder in the polymer backbone can exist in the form of chemical defects, backbone torsion or coiling20–22 and this can affect the p-system conjugation length and the resulting spectroscopic properties. For C-PPV, the hydrophobic backbone self-coils in water to reduce polymer–solvent interaction, breaking the p-system conjugation length and promoting self-quenching.23 Spectroscopic changes similar to those observed for C-PPV complexed with DNA, including PL enhancement, could be reproduced when C-PPV was dissolved in a mixed water–dimethyl sulfoxide (DMSO) solution, with DMSO being a better solvent for C-PPV than water (see Table 1 and Fig. S4, ESI†). Thus, this confirms the hypothesis of a polymer chain conformational change for C-PPV when complexed with DNA. Next to electrostatic attraction, leading to a polymer chain conformational change in the case of C-PPV complexed with DNA, hydrophobic interactions also define the resulting spectroscopic properties of the complex, as suggested for other heterocyclic aromatic conjugated polyelectrolytes interacting with DNA.10,18 Complexation between C-PPV and DNA strongly perturbs the right-hand stacking of DNA bases as shown by CD spectra (Fig. S2, ESI†), suggesting strong hydrophobic interactions between the C-PPV’s backbone and DNA bases. For C-PPV, hydrophobic interactions occur most probably as p–p stacking between the C-PPV’s phenylene vinylene backbone and DNA bases. p–p stacking increases the p-conjugation system, here
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next to C-PPV’s polymer chain unfolding induced by the electrostatic attraction with the DNA’s phosphate backbone. As such, both interactions will contribute to the observed red shift of the absorption spectrum of the polymer (Fig. 2 and Table 1, Dlabs). Assuming that the electrostatic interaction between C-PPV’s charged side groups and the DNA’s phosphate backbone is similar among different sequences, the contribution of this particular interaction to the overall red shift of the absorption spectrum should be similar among different DNA sequences complexed with C-PPV. As such, the magnitude of the observed red shift in absorption for a given C-PPV–DNA complex (Table 1, Dlabs) quantifies the strength of the p–p stacking between the C-PPV’s backbone and DNA bases. The largest spectral shift in absorption, e.g. strongest p–p stacking, occurs for C-PPV–ssDNAd(C)25 (58 nm), followed by ssDNAd(T)25 (43 nm), ssDNAd(A)25 (38 nm) and ssDNAd(G)25 (18 nm). From these absorption spectral shifts we hypothesize that the C-PPV’s phenylene vinylene backbone, a six member aromatic ring, interacts stronger with single (six-member) ring pyrimidines (C,T bases) than with double ring pyrimidines (C,G bases). These findings are in contrast to those previously reported for cationic polythiophenes interacting with DNA where stronger stacking was observed with purines.11 This is not controversial because a five-membered aromatic ring like thiophene might prefer stacking with a homologous ring that is present only in purines. C-PPV–ssDNAd(G)25 behaves differently than all other complexes, featuring small spectral shift in absorption and quenched PL. G oligonucleotides are known to form secondary structures24 which in this case might prevent p–p stacking with C-PPV. As such, in a C-PPV–ssDNAd(G)25 complex, interaction between C-PPV and DNA will be purely electrostatic attraction, resulting in turn in the smallest observed red shift in absorption (Fig. 2i and Table 1). We demonstrated DNA sequence specific PL enhancement of a conjugated polyelectrolyte complexed with homo and hetero oligonucleotides and discussed it as resulting from an interplay between electrostatic and hydrophobic interactions. Electrostatic attraction between polymer’’s charged side groups is essential in promoting hydrophobic interactions responsible for the observed PL enhancement, here in the form of p–p stacking between the polymer backbone and the DNA bases. Further engineering of such conjugated polyelectrolytes might provide complexes with tunable PL and chiroptical properties that can be useful in the development of biosensors with sequence specific recognition. Research was carried out at the Center for Functional Nanomaterials, Brookhaven National Laboratory supported by
This journal is © The Royal Society of Chemistry 2014
Communication
the U.S. Department of Energy, Office of Basic Energy Sciences, Contract No. DE-AC02-98CH10886 (M.C., Z.L.) and by the Biomaterials Program of the Division of Materials Science and Engineering, Office of Basic Energy Sciences (H.L.W.). We thank Fang Lu from Brookhaven Laboratory and Prahlad K. Routh from Stony Brook University for help with some of the experiments reported herein.
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