Assembly of neutral conjugated polymers with layered double hydroxide nanosheets by the layer-by-layer method

Abstract

Layer-by-layer (LbL) assembly is a simple and prevalent method for constructing functional multilayer thin films with charged components and is usually based on electrostatic interactions. This article reports neutral conjugated polymers (NCPs) that can also be assembled with exfoliated MgAl-layered double hydroxide (LDH) nanosheets to form the ordered inorganic/organic hybrid ultrathin films (UTFs) via the LbL assembly technique. Furthermore, neutral small molecules such as PCBM could also be co-assembled with LDH nanosheets by being encapsulated into the NCPs during the LbL process. These composite UTFs were characterized thoroughly and were found to have an ordered periodic structure along the film normal with a 7.6–13.6 nm thickness of the interlayer space, the assembled bilayers numbers could be up to 40, and the interlayer polarity was transfered to hydrophobic ones, which facilitated the assembly of some organic functional molecules. The driving force for this assembly is discussed and attributed to the interactions between the positive charge of LDH nanosheets and the delocalized π electrons of these NCPs. Moreover, the (NCPs/LDH)_n UTFs could detect some small biological medicine molecules such as protoporphyrin, which has the potential to be a novel type of biological fluorescence sensor material.

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Introduction

The layer-by-layer (LbL) electrostatic assembly technique is a simple and prevalent method for constructing functional multilayer thin films and is usually done using charged polymers. Principally, LbL assembly is a cyclic process in which a charged species is adsorbed onto a substrate, and an oppositely charged species is absorbed on top of the first layer through electrostatic interactions. This prepares a single bilayer with a thickness generally on the order of nanometer magnitude, and the deposition process can then be repeated for many times until a multilayer film of desired thickness has formed. Except for electrostatic interactions,^1,2 other molecular interactions (e.g., covalent bonding,^3,4 coordination bonding,^5,6 charge transfer interaction,^7,8 hydrogen bonding,^9–12 DNA hybridization,^13,14 and streptavidin–biotin binding^15,16) are now well established for LbL assembled films with diverse species (e.g., polymers, nucleic acids, proteins, lipids, and nanoparticles). Due to some extraordinary advantages, such as ease of preparation, versatility and fine control over the film structure and properties, these LbL films are used widely in a range of various fields.

Layered double hydroxides (LDHs) are a large class of layered inorganic materials that can be described by the general formula [M^II_1−xM^III_x(OH)₂]^z+Aⁿ⁻_z/n·yH₂O (M^II and M^III are divalent and trivalent metals, respectively; Aⁿ⁻ is the anion).^17,18 As a typical anionic inorganic layered material, LDHs laminates can be exfoliated into LDHs nanosheets,^19–21 which provide the building blocks for the construction of multifunctional composite ultrathin film (UTF) materials. Based on the electrostatic interactions, our previous works have realized the assembly of functional polyanion with LDHs nanosheets^22,23 and developed a co-assembly method to assemble the LDH nanosheets with small anion/cation and polyanion blends,^24,25 and even neutral small molecules after being incorporated into the block copolymer micelles.^26,27 Based on hydrogen bonding interactions, the assembly of neutral polymer tethering –NH₂ or –OH group with LDHs nanosheets has been realized,²⁸ and the co-assembly of the LDHs nanosheets with neutral organic small molecule/neutral metal complexes and neutral polymer blends were developed.^29,30 At the same time, the assembly of some complex biological molecules (such as proteins and nucleic acids) with LDHs nanosheets were also realized.^31,32

Conjugated polymers (CPs) have attracted considerable attention due to their potential applications for light-emitting diodes (LEDs),³³ solar cells,^34–36 photovoltaic devices,³⁷ field-effect transistors,³⁸ and biomedicine.^39,40 Traditionally, optoelectronic device with CPs films can be fabricated by spin-coating or ink-jet printing techniques.^41–43 However, these methods are environmentally contaminative, costly, complicated and not adaptable to fabricate CPs film over a large area,^44–46 which limits the wide application of CPs films. On the other hand, the property regulation of CPs was restricted to chain modification that depends on the chemical synthesis. Therefore, a simple and flexible route for regulating the properties of CPs is imperative for its material applications. The LBL technique is a convenient method to prepare UTFs and if this method could be used to prepare CPs UTFs, it would extend to wide application of CPs.

In our previous studies,^30,31 the neutral conjugated polymer (NCP), poly(vinyl carbazole) (PVK) (Scheme 1), was assembled with LDH nanosheets to form (PVK/LDH)_n UTFs by a hydrogen-bonding LbL assembly method. Furthermore, some neutral small molecules (perylene and tetraphenylethylene) blended with PVK were successfully introduced into the interlayer of the LDH nanosheets. In this study, a group of special neutral conjugated polymers (NCPs) (PFH-Ec, PFPC, P3HT, PPE, PHF, Scheme 1) was selected and used to assemble with LDHs nanosheets to form the multilayer UTFs via the LbL technique. The obtained (NCPs/LDH)_n UTFs showed a long-rang ordered stacking structure and held well-defined fluorescence properties, originating from the NCPs. These UTFs could detect some small biological medicine molecules such as protoporphyrin, which showed it has the potential to be a novel type of biological fluorescence sensor materials. Furthermore, neutral small molecules (such as PCBM) accompanied by these NCPs (P3HT, PFPC or PFH-Ec) could also be assembled with LDH nanosheets to form composite UTFs. This work not only provides a feasible route to construct organic–inorganic composite film materials, but also greatly broadens the application range of the LbL assembly technique.

Scheme 1 The molecular structure of PFH-Ec, PFPC, P3HT, PPE, PHF, PVK and PCBM.

Experimental section

Reagents and materials

Analytical grade Mg(NO₃)₂·6H₂O, Al(NO₃)₃·9H₂O, NaNO₃, HNO₃, formamide, chlorobenzene, toluene, tetrahydrofuran, ethanol, urea, NH₃·H₂O, H₂O₂, H₂SO₄, and NaOH were purchased from Beijing Chemical Co. Ltd. Poly-(dimethyldiallylammonium chloride) (PDDA, M_w = 100 000–200 000), poly(2,5-di(2′-ethylhexyl)phenylene-1,4-ethynylene) (PPE), poly(9,9-di-n-hexylfluorenyl-2,7-diyl) (PHF), poly(9,9-n-diylhexyl-2,7-fluorene-alt-9-phenyl-3,6-carbazole) (PFPC, average M_w = 9195), and poly[(9,9-dihexylfluoreny-2,7-diyl)-co-(9-ethylcarbazole-2,7-diyl)] (PFH-Ec, M_w > 20 000) were purchased from Sigma-Aldrich Chemical. Co. Ltd. Poly(3-hexylthiophene-2,5-diyl) (P3HT, M_w = 30 000–60 000) and [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM, >99%, HPLC) were purchased from Xi'an polymer light technology Corp., and poly(styrene sulfonic acid) (PSS, M_w = 70 000) was purchased from J&K Chemical. Co. Ltd. All these reagents were used without further purification. Deionized and decarbonated water was used in all the experiments. Ultrapure water was prepared by a Millipore Ultrapure Water Purifier from RephiLe Bioscience, Co. Ltd.

Preparation of MgAl-LDH nanosheets and (NCPs/LDH)_n UTFs

The process of synthesis and exfoliation of MgAl-LDH (Mg/Al = 2 : 1) was similar to that described in previous works.^47,48 Mg₂Al-LDH (0.1 g) was shaken in 100 mL of formamide solution for 24 h to produce a colloidal suspension of exfoliated Mg₂Al-LDH nanosheets. The NCPs (PPE, PHF, PFPC, PFH-Ec and P3HT) were dissolved in appropriate solvents based on their solubility, PPE and PHF were dissolved in tetrahydrofuran and PFPC, PFH-Ec and P3HT were dissolved in toluene to ensure the concentration was 0.01 g L⁻¹. The PCBM@NCPs solutions were prepared by blending the PCBM solution and P3HT, PFH-Ec or PFPC solution with the concentration ratios of P3HT : PCBM = 0.5 g L⁻¹ : 1.25 g L⁻¹, PFH-Ec : PCBM = 0.01 g L⁻¹ : 0.1 g L⁻¹, PFPC : PCBM = 0.01 g L⁻¹ : 0.05 g L⁻¹, respectively.

The quartz substrates were cleaned in a mixed solution of concentrated H₂SO₄/30% H₂O₂ (v/v = 7 : 3) for 30 min and then washed by anhydrous alcohol and deionized water thoroughly. To improve the adhesion between the film and the substrate, the cleaned substrates were dipped into a cationic PDDA solution (10 g L⁻¹) for 30 min and thoroughly rinsed with distilled water and dried in a nitrogen gas flow. Then, the substrates were dipped into an anionic PSS solution (10 g L⁻¹) for 30 min and thoroughly rinsed with distilled water and dried in a nitrogen gas flow. This treatment made the quartz substrates pre-assembled with one layer polymer UTF.

The preparation of the (NCP/LDH)_n multilayer UTFs was carried out as follows: the pretreated substrates were immersed alternatively into the formamide colloidal suspension containing exfoliated MgAl-LDH nanosheets (1 g L⁻¹) for 10 min and a NCP solution (0.01 g L⁻¹) for 10 min, respectively. After each immersion in LDHs colloidal suspension, the substrates were thoroughly washed with distilled water and dried in a nitrogen gas flow at room temperature. After each immersion in the NCP solution, the substrates were thoroughly washed with anhydrous alcohol and dried naturally at room temperature.

Sample characterization

UV-Vis absorption spectra were obtained on a Shimadzu U-3600 spectrophotometer with a 1.0 nm slit width. Fluorescence spectra were obtained on a RF-5301 PC fluorospectrophotometer with identical parameters. The polarized fluorescence spectra were obtained with an Edinburgh Instruments' FL 900 fluorimeter. Small-angle XRD patterns of the films were obtained with a Rigaku 2500VB2+ PC diffractometer using Cu Kα radiation (λ = 1.541844 Å, 2θ = 0.5–8°) at 40 kV, 50 mA, with the step-scanned mode in 0.04° (2θ) per step and count time of 10 s per step. A scanning electron microscope (SEM Zeiss Supra 55) was used to investigate the morphology and thickness of the (NCPs/LDH)_n UTFs. Atomic force microscope (AFM) software (Digital Instruments, Version 6.12) was used to obtain the surface roughness data of the UTFs.

Results and discussion

Assembly of (NCPs/LDH)_n UTFs

Take the (PFH-Ec/LDH)_n UTF as an example, Fig. 1A shows the UV-Vis absorption spectra of the (PFH-Ec/LDH)_n UTFs with various bilayer numbers n deposited on quartz substrates. The intensities of the absorption bands at 215, 237 and 379 nm correlated nearly linearly with the number of bilayers n (Fig. 1A, inset), which suggests a stepwise and regular deposition procedure with almost equal amounts of PFH-Ec polymers and LDH nanosheets incorporated in each cycle. Compared with the absorption spectra of pristine PFH-Ec solution (Fig. S1A, ESI†), the absorption band of the (PFH-Ec/LDH)_n UTFs showed a small blue shift, which may be attributed to the weak interaction between PFH-Ec molecules and LDH nanosheets.

Fig. 1 Characterization of the assembly of (PFH-Ec/LDH)_n (n = 5–20) UTFs: (A) UV-Vis absorption spectra (inset: plots of the absorbance at 215, 237 and 379 nm versus n); (B) fluorescence spectra with 380 nm excitation (inset: images of UTFs with different n exposed to 365 nm ultraviolet light).

The fluorescence spectrum of (PFH-Ec/LDH)_n (n = 5–20) UTFs is shown in Fig. 1B. The intensities of the sharp luminescence peaks at 420 and 444 nm of the (PFH-Ec/LDH)_n UTFs (n = 5–20) also display a monotonic increase along with n, and the fluorescent brightness gradually increased under the 365 nm ultraviolet light (Fig. 1B, inset), which further confirms that the PFH-Ec content increased linearly with the number of bilayer n. This implies that the PFH-Ec polymer within the interlayers of LDHs was distributed homogenously. Compared with the fluorescence spectrum of pristine PFH-Ec solution (Fig. S1B, ESI†), the maximum emission band of the (PFH-Ec/LDH)_n UTFs showed a 4–6 nm red shift, which could be attributed to the specific 2D alignment effect of PFH-Ec polymers within the LDH nanosheets.

SEM and AFM observation were used to explore the morphology of the (PFH-Ec/LDH)_n UTFs. The side view of SEM images (Fig. 2A) show that the thickness of the (PFH-Ec/LDH)_n (5–20) UTFs approximately linearly increase upon increasing the bilayer number, and the average thickness of one bilayer of PFH-Ec/LDH was estimated to be 11.3 nm, which is approximately close to the basal spacing observed by XRD (11.9 nm) (Fig. 2B, 2θ = 0.74°). This further confirms that the (PFH-Ec/LDH)_n UTFs presented a uniform and periodic stacking layered structure, in agreement with the linear growth behaviors revealed by the optical absorption and fluorescence spectra. The AFM images (Fig. 3) showed that the surface of the UTFs was uniform and continuous, and the root-mean-square (RMS) roughness of the (PFH-Ec/LDH)_n UTFs increased with the increasing bilayer number n over a 2 μm × 2 μm area.

Fig. 2 (A) The SEM side view images of the (PFH-Ec/LDH)_n (n = 5–25) UTFs. The scale is constant in all images. (B) Small angle XRD pattern of the (PFH-Ec/LDH)₄₀ UTF.

Fig. 3 The AFM images of the (PFH-Ec/LDH)_n (n = 5–25) UTFs (inset: 3D maps corresponds the 2D images), scanning area was randomly chosen and is 2 μm × 2 μm. RMS: root-mean-square roughness.

To investigate the microenvironment of the assembled NCPs between the LDH nanosheets of the UTFs, the polarized luminescence spectra of the UTFs were obtained. In contrast to the PFH-Ec solution (r = 0.02) or the PFH-Ec spin-coated film (r = 0.01) with poor luminescence polarization, the (PFH-Ec/LDH)₁₀ UTF displayed well-defined luminescence polarization (r = 0.10, Fig. 4), indicating an improvement in the overall orientation of the PFH-Ec between the LDH nanosheets. It can be speculated that the host–guest interaction induced the PFH-Ec to be arranged into an oriented manner, which may be responsible for the enhanced polarized photoemission.

Fig. 4 Polarized fluorescence profiles for the VV, VH, HH, HV modes and anisotropic value (r) for the (A) PFH-Ec solution, (B) PFH-Ec spin-coating thin film and (C) (ANS/LDH)₁₀ UTF.

Apart from (PFH-Ec/LDH)_n UTFs, a series of other (NCPs/LDH)_n UTFs were also assembled successfully such as (P3HT/LDH)_n, (PPE/LDH)_n, (PHF/LDH)_n, and (PFPC/LDH)_n UTFs. Fig. S2 in the ESI† shows the optical absorption spectra of (P3HT/LDH)_n, (PFPC/LDH)_n, (PHF/LDH)_n, (PPE/LDH)_n UTFs and the characteristic absorption peaks of these films were consistent to the corresponding solution, and the absorption intensity increased along with the bilayer number n, which suggests that a stepwise and regular deposition procedure with almost equal amounts of NCPs and LDH nanosheets incorporated in each cycle. Compared with the absorption spectra of the pristine sample solution, the characteristic absorption bands of the (P3HT/LDH)_n, (PHF/LDH)_n, (PPE/LDH)_n UTFs showed an obvious red shift, which may be attributed to the 2D alignment effect of the guest molecule between the LDH nanosheets. The fluorescence intensity of these (NCPs/LDH)_n UTFs also displays a monotonic increase along with n (Fig. S3, ESI†), which further confirms that the NCPs were successfully assembled with the LDH nanosheets. The small XRD patterns of (P3HT/LDH)_n, (PFPC/LDH)_n, (PHF/LDH)_n, and (PPE/LDH)_n UTFs (Fig. S4, ESI†) suggest that these films also have a periodically layered structure along the UTFs normal direction, and further confirms that the (NCPs/LDH)_n UTFs presented a uniform and ordered layered structure, in agreement with the linear growth behaviors revealed by the absorption and fluorescence spectra.

Assembly of (PCBM@NCPs/LDH)_n UTFs

Moreover, in addition to neutral conjugated polymers, neutral small molecules could also be encapsulated within the interlayers between LDH nanosheets. Based on the co-assembly method,^11,14 (PCBM@PFH-Ec/LDH)_n, (PCBM@PFPC/LDH)_n and (PCBM@P3HT/LDH)_n UTFs were constructed by alternative assembly of neutral small molecule (PCBM) and NCPs (PFH-Ec, PFPC or P3HT) blend with LDH nanosheets. It was found that the optimal concentration ratios of the NCPs@PCBM mixture were 0.01 : 0.1 g L⁻¹ for PFH-Ec : PCBM, 0.01 : 0.05 g L⁻¹ for PFPC : PCBM and 0.5 : 1.25 g L⁻¹ for P3HT : PCBM. The characteristic absorption peaks of (PCBM@PFH-Ec/LDH)_n, (PCBM@PFPC/LDH)_n and (PCBM@P3HT/LDH)_n UTFs were consistent with that of the corresponding solution (Fig. S2 and S5, ESI†), and the absorption intensity increased along with the bilayer number n (Fig. 5), which confirmed that both the PCBM and NCPs (PFH-Ec, PFPC or P3HT) were successfully assembled.

Fig. 5 UV-Vis absorption spectra of (A) (PCBM@PFH-Ec/LDH)_n (n = 5–20), (B) (PCBM@PFPC/LDH)_n (n = 5–20) and (C) (PCBM@P3HT/LDH)_n (n = 5–40) UTFs. The insets show the plots of absorbance versus n.

The morphology and structure of the (PCBM@P3HT/LDH)_n UTFs were studied by SEM, AFM and XRD. The side view of SEM images (Fig. S6, ESI†) shows that the thickness of the (PCBM@P3HT/LDH)_n (n = 5–20) UTFs approximately linearly increased with the bilayer number n, and the average thickness of one bilayer of PCBM@P3HT/LDH UTF could be estimated to be 7.5 nm, which is approximately close to the basal spacing observed by XRD (ca. 7.7 nm) (Fig. S8A, ESI†). This further confirms that the (PCBM@P3HT/LDH)_n UTFs presented a uniform and periodic layered structure. The top view of the AFM images (Fig. S7, ESI†) shows that the surface of the UTFs was microscopically continuous and smooth.

PCBM is a typical organic electron acceptor with good electron mobility and it is easy to realize electron transfer between PCBM and a substance rich with electrons. The NCPs in our work had strong delocalized electrons and their fluorescence was easily quenched by blending with the electron acceptor PCBM, such as P3HT@PCBM, which is a typical electron donor–acceptor pair and widely studied in the literature.^58,59 Our experiments also indicated that they all exhibit fluorescence quenching in the PFH-Ec@PCBM, PFH@PCBM, PFPC@PCBM, PPE@PCBM and P3HT@PCBM system. Taking the (PCBM@PFH-Ec/LDH)_n and (PCBM@PFPC/LDH)_n UTFs as examples, the fluorescence intensity of (PCBM@NCPs)_n UTFs was distinctly weaker than the corresponding (NCPs/LDH)_n UTFs (Fig. 6), indicating that the PCBM as an electron acceptor effectively quenched the fluorescence of PFH-Ec and PFPC, which illustrates the formation of a two-dimensional electron donor–acceptor system in the interlayers of the LDH nanosheets, and provides the 2D models for research of the 2D photoelectric conversion effect.

Fig. 6 Fluorescence spectra of (A) (PFH-Ec/LDH)_n (n = 5–20) and (PCBM@PFH-Ec/LDH)_n (n = 5–20), (B) (PFPC/LDH)_n (n = 5–20) and (PCBM@PFPC/LDH)_n (n = 5–20) UTFs.

The thickness of NCPs/LDH UTFs

In general, NCPs are prone to aggregate due to the intermolecular π–π interaction or hydrophobic interaction, which are detrimental for their fluorescence properties and restrict their broader applications in solid state devices. Table 1 shows the film thickness of the assembly NCPs/LDH UTFs derived from small angle XRD data, the bilayer thickness of the UTFs was about 9.1–13.5 nm, whereas the thickness of the LDH monolayer was 0.48 nm, thus the thickness of the interlayer NCPs was 8.62–13.02 nm. The thickness of the NCPs monolayer was about 0.488–0.540 nm (Table 1) determined with ChemBio3D software and with consideration of the van der Waals radius r_w (the C atom r_w is 0.172 nm) between the adjacent molecules, thus the NCPs between the LDHs monolayers were estimated to arrange between 16 and 24.3 monolayers, if the NCPs were assumed to extend into the polymer chain (taking the PHF/LDH UTF as an example, the structure model of LDH/PHF/LDH is showed in Scheme S1, ESI†), which showed that the LDH nanosheets can effectively prevent the aggregation of NCPs to a great extent. The NCPs can be assembled with LDH nanosheets alternatively on the substrate and this can be repeated up to at least 40 times, the UTF-40 still shows a well-defined periodic structure. Compared with the NCPs/LDH monolayer, the (PCBM@NCPs/LDH) UTFs were thinner per bilayer. At the same time, the characteristic absorbance values of the (PCBM@NCPs/LDH)_n UTFs are not lower than that of the (NCPs/LDH)_n UTFs with the same bilayers (Fig. 1A, 4 and S2 in the ESI†), thus it is impossible that the existence of PCBM would reduce the quantity of NCPs in the (PCBM@NCPs/LDH)_n UTFs. This result indicates the involvement of PCBM is favorable for the dense interlayer distribution of NCPs, which may attributed to the hydrophobic nature of PCBM molecules to some extent.

Table 1 The small angle XRD 2θ and d value for different (NCPs/LDH)₄₀ and (NCPs/LDH)₄₀ UTFs^a

UTFs	n (max)	2θ/degree	d/nm	dinter/nm	Llateral/nm	Nlayer
a d: bilayer of the UTFs, dinter: thickness of the interlayer NCP, Llateral: thickness of NCP, Nlayer: estimated layer numbers of the NCPs between LDH layers.
(PFH-Ec/LDH)n	40	0.74	11.9	11.42	0.534	21.4
(PFPC/LDH)n	40	0.76	11.6	11.12	0.540	20.6
(P3HT/LDH)n	40	0.97	9.10	8.62	0.536	16.0
(PPE/LDH)n	40	0.91	9.70	9.22	0.488	18.9
(PHF/LDH)n	40	0.65	13.5	13.02	0.534	24.3
(PCBM@PFH-Ec/LDH)n	40	1.06	8.32	7.84	0.992	7.9
(PCBM@PFPC/LDH)n	40	1.30	6.79	6.31	0.995	6.3
(PCBM@P3HT/LDH)n	40	1.15	7.68	7.20	0.993	7.2

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The interactions between NCPs and LDHs nanosheets

All the NCPs used for LbL assembly with LDH nanosheets had some common structural features: neutral linear polymer with chain configurations,^35,49 no net charge, and no modification groups such as –OH and –NH₂. Therefore, this excludes that the electrostatic interaction plays a dominant role for the assembly between NCPs and LDH nanosheets as the traditional LbL assembly, which may be related to the weak hydrogen-bonding interaction or van der Waals (VDW) forces (that is dispersive, orientational, and inductive). The molecules such as PFH-Ec and PFPC have highly electronegative N heteroatoms, thus it is possible for these molecules to assemble with LDH nanosheets partly based on the N⋯H–O hydrogen-bond interaction, as reported in our previous work.³⁰ However, the other molecules such as PPE, PHF and P3HT, neither of which has net charge nor highly electronegative heteroatoms, so it is impossible for them to exert a hydrogen-bond interaction with the LDH nanosheets, and the assembly driving force may be related to the VDW interactions. Because of their unique π electron conjugated system, the NCPs molecules have great deformation ability and polarization rate, weak VDW interactions existing between the LDHs nanosheets and each aggregation unit of the NCPs, although there is no strong electrostatic interactions. Thus, the VDW forces should be accumulated to drive the assembly of LDHs nanosheets with NCPs. Furthermore, the VDW force is an inducible force because the strong delocalized electrons flow across the NCP molecular chain can produce inducible electrostatic interactions under the positive charges of the LDHs laminates, which can drive the assembly of NCPs and LDHs nanosheets.

Overall, there are two types of possible interactions between the NCPs and LDH nanosheets: hydrogen-bond interaction and induced VDW interaction based on delocalized π electrons. Both the two types of interactions may exist between the LDH nanosheets and the NCPs such as PFH-Ec and PFPC with electronegative heteroatoms N, and the latter exists between the LDH nanosheets and the NCPs such as PPE, PHF and P3HT, neither of which has net charge, nor highly electronegative heteroatoms. Based on the results mentioned above, we speculate that as long as the molecules have extended dipole moments such as NCPs, it is possible to assemble with LDHs nanosheets by the LbL method. Therefore, this implies that numerous functional organic polymers with delocalized electrons can be assembled with LDH nanosheets.

Apart from the hydrogen-bond and weak VDW interactions that combined the NCPs with LDH nanosheets, it cannot be excluded that there were some other anions to completely balance the excess positive charges on the LDH nanosheets in the UTFs. In the assembly process, it can be expected that the charge-balanced anions should be CO₃²⁻ anions coming from the ambient CO₂ and some NO₃⁻ anions from the exfoliated LDH suspension during the assembly process under ambient conditions. Experimentally, the LbL assembly process of NCPs with LDH nanosheets can sustain for more than 40 cycles, which further verifies the feasibility of the weak interaction assembly for the neutral molecules accompany by the anions CO₃²⁻/NO₃⁻ to balance the positive charges of LDH nanosheets.

Interlayer polarity of the (NCPs/LDH)_n UTFs

It is known that for the CO₃²⁻ or NO₃⁻ intercalated LDHs, the interlayer was a hydrophilic environment, thus some hydrophobic neutral functional molecules (such as small organic molecules, polymers, or proteins and other biological macromolecules) were difficult to be introduced into the interlayers of LDHs by a simple intercalation reaction or electrostatic assembly. The successful assembly of the NCPs with LDHs converted the hydrophilic interlayer to a hydrophobic one, which realized the polarity inversion in the interlayers and obtained a lipophilic hydrophobic environment to facilitate the introduction of other neutral organic molecules. For example, PCBM is a small neutral organic molecule, which is difficult to assemble with LDH nanosheets, but when it is blended with NCPs (such as P3HT, PFPC or PFH-Ec), it can be easily assembled with LDH nanosheets. The hydrophobic interlayer is a nanometer scale 2D space, which facilitates the fabrication of 2D composite system and the study of the potential 2D nanometer effect. It can be expected that this polarity inversion by NCP assembly can be used to construct numerous novel inorganic/organic hybrid layered composites, which have a novel structure and potential performance.

Fluorescent detection of protoporphyrin with the (NCPs/LDH)_n UTFs as a biosensor

Recently, CPs were widely used in the field of biosensors since they provide homogeneous and manageable film characteristics, stability, biocompatibility, reproducibility, and ease of production.^40,50 Fluorescent CPs, with plenty of absorbance repeat units possess extremely strong light-harvesting ability, which make them easy to respond to electron-rich bioanalytes.^39,51 As an example of bioactive small molecules, protoporphyrin is a main ingredient of hemoglobin, which is critically important in oxygen transportation-associated energy metabolism and various inflammatory lesions.^52,53 At the same time, protoporphyrin plays a significant role in the biological and photophysical field due to the special optical properties from their extended π-conjugated electronic macrocyclic structures.^54,55

Herein, we studied the protoporphyrin detection by the (NCPs/LDH)_n UTFs and found that the UTFs exhibited a significant decrease in fluorescent intensity upon increasing the protoporphyrin concentration. Taking (PFH-Ec/LDH)₈, (PPE/LDH)₈ and (PHF/LDH)₈ UTFs as examples (Fig. 7), the fluorescence intensity of the UTFs reduced with the increase in the protoporphyrin concentration, and they showed a good linear relationship at low protoporphyrin concentration (about 0–10 μg mL⁻¹) (Fig. 7B). Furthermore, the UTFs displayed reversible fluorescence response between protoporphyrin (pH = 7) and without protoporphyrin (pure water) (Fig. 7C), and after five cycles by alternating treatment with protoporphyrin solution and pure water, the film still presented strong fluorescent emission, confirming the reusability of the (NCP/LDH)_n UTFs for detecting protoporphyrin, which indicated that this film can be a novel type of biological sensor material.

Fig. 7 Fluorescence spectra of (A) (PFH-Ec/LDH)₈, (B) (PPE/LDH)₈ and (C) (PHF/LDH)₈ UTFs with protoporphyrin solution (pH = 7) of different concentrations, respectively. Plots of fluorescence intensity at the maximum emission wavelength versus protoporphyrin concentration: (D) 422 nm corresponding to (PFH-Ec/LDH)₈ UTF, (E) 435 nm corresponding to (PPE/LDH)₈ UTF, (F) 423 nm corresponding to (PHF/LDH)₈ UTF. Fluorescence response of protoporphyrin over five consecutive cycles: (G) (PFH-Ec/LDH)₈, (H) (PPE/LDH)₈ and (I) (PHF/LDH)₈ UTFs, respectively.

It is speculated that the detection mechanism of (NCP/LDH)_n UTFs for protoporphyrin is attributed to the inner-filter effect (IFE) of the fluorescence quenching of the (NCPs/LDH)_n UTF by the protoporphyrin molecules. The IFE refers to a phenomenon of fluorescence quenching, resulting from the absorption of the excitation and/or emission light of fluorophores by absorbers in the detection system.^56,57 In this system, the (NCPs/LDH)_n UTFs were used as a fluorescence sensor and the protoporphyrin was used as a fluorescence quencher. Because the absorption of protoporphyrin is in a relatively wide range (320–540 nm) (Fig. S9 in the ESI†), which almost includes the excitation and emission wavelength of the UTF, IFE is possible between the UTF and protoporphyrin. The fluorescence intensity of the UTFs was quenched obviously with the increasing protoporphyrin concentration, which further proves the quenching was based on IFE.

Conclusions

In summary, a series of NCPs, such as PPE, PHF, PFPC, P3HT and even the small neutral optoelectronic functional molecule PCBM, were successfully assembled with positively charged LDHs nanosheets. The assembly driving forces between the LDHs nanosheets and the NCPs were analysed and were attributed to inductive VDW interactions of delocalized π electrons of NCPs by the positive charges of the LDH nanosheets, which were different from pure electrostatic and hydrogen-bonding interactions, and breaking the traditional limit that positive-charged LDHs could only be assembled with anions or neutral molecules with –OH, –NH₂ groups. Furthermore, the (NCPs/LDH)_n UTFs could detect some small biological medicine molecules such as protoporphyrin. These results expand the assembly scope and applications of organic functional materials. The assembled UTFs had an ordered periodic structure with nanometer sized hydrophobic interlayers and the NCPs incorporation converted the hydrophilic environment into a hydrophobic one, which is favorable for further encapsulating other neutral organic functional molecules for constructing novel 2D composite UTFs. At the same time, this study provides an experimental base for the LDHs applications in the field of conjugated polymer molecule photovoltaic devices and biosensors.

Acknowledgements

This study was supported by the 973 Program (grant no. 2014CB932101), the National Natural Science Foundation of China, 111 Project (grant no. B07004), the Program for Changjiang Scholars, the Innovative Research Team in University (IRT 1205) and the Central University Research Funds of China (buctrc201527).

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Footnote

† Electronic supplementary information (ESI) available: UV-Vis absorption spectra and fluorescence emission spectra of the PFH-Ec solution. UV-Vis absorption spectra of P3HT, PFPC, PHF, PPE solution and (P3HT/LDH)_n (n = 4–20), (PFPC/LDH)_n (n = 5–20), (PHF/LDH)_n (n = 5–20), (PPE/LDH)_n (n = 5–25) UTFs. Fluorescence spectra of PFPC, PHF, PPE solution and (PFPC/LDH)_n (n = 5–20), (PHF/LDH)_n (n = 5–20), (PPE/LDH)_n (n = 5–25) UTFs. Small angle XRD patterns of (P3HT/LDH)₄₀, (PFPC/LDH)₄₀, (PHF/LDH)₄₀, (PPE/LDH)₄₀ UTFs. UV-Vis absorption spectra of PCBM solution. The SEM side view images of the (PCBM@P3HT/LDH)_n (n = 5–25) UTFs. The AFM images of the (PCBM@P3HT/LDH)_n (n = 10, 15) UTFs, scanning area is random for 2 μm × 2 μm. Small angle XRD patterns of the (PCBM@PFH-Ec/LDH)₄₀, (PCBM@PFPC/LDH)₄₀ and (PCBM@P3HT/LDH)₄₀ UTFs. The structure model of LDH/PHF/LDH UTF. Absorption spectra of porphyrin solution (pH = 7). See DOI: 10.1039/c6ra17924j

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