Elaborately engineering of a dual-drug co-assembled nanomedicine for boosting immunogenic cell death and enhancing triple negative breast cancer treatment

Pure drug-assembled nanosystem provides a facile and promising solution for simple manufacturing of nanodrugs,whereas a lack of understanding of the underlying assembly mechanism and the inefficient and uncontrollable drug release still limits the development and application of this technology.Here,a simple and practical nanoassembly of DOX and DiR is constructed on basis of their co-assembly characteristics.Multiple interaction forces are found to drive the co-assembly process.Moreover,DOX release from the nanoassembly can bewell controlled by the acidic tumormicroenvironment and laser irradiation,resulting in favorable delivery efficiency of DiR and DOX in vitro and in vivo.As expected,the nanoassembly with high therapeutic safety completely eradicated the mice triple negative breast cancer cells(4T1)on BALB/c mice,owing to synergistic chemo-photothermal therapy.More interestingly,DiR and DOX synergistically induce immunogenic cell death(ICD)of tumor cells after treatment,enabling the mice to acquire immune memory against tumor growth and recurrence.Such a facile nanoassembly technique provides a novelmultimodal cancer treatment platform of chemotherapy/phototherapy/immunotherapy.

General synthesis of hollow mesoporous conducting polymers by dual-colloid interface co-assembly for high-energy-density micro-supercapacitors

Rational design and precise regulation over the morphology, structure, and pore size of functional conducting mesoporous polymers with enriched active sites and shorten electron–ion transport pathway are extremely important for developing high-performance micro-supercapacitors (MSCs), but still remain a great challenge. Herein, a general dual-colloid interface co-assembly strategy is proposed to fabricate hollow mesoporous polypyrrole nano-bowls (mPPy-nbs) for high-energy-density solid-state planar MSCs. By simply adjusting the size of block copolymer micelles, the diameter of polystyrene nanospheres and the amount of pyrrole monomer, mesopore size of the shell, void and shell thickness of mPPy-nbs can be simultaneously controlled. Importantly, this strategy can be further utilized to synthesize other hollow mesoporous polymers, including poly(tris(4-aminophenyl)amine), poly(1,3,5-triaminobenzene) and their copolymers, demonstrative of excellent universality. The structurally optimized mPPy-nb exhibits high specific surface area of 122 m^(2) g^(−1)and large capacitance of 225 F g^(−1) at 1 mV s^(−1). Furthermore, the MSCs assembled by mPPy-nbs deliver impressive volumetric capacitance of 90 F cm^(−3) and energy density of 2.0 mWh cm^(−3), superior to the most reported polymers-based MSCs. Also, the fabricated MSCs present excellent flexibility with almost no capacitance decay under varying bending states, and robust serial/parallel self-integration for boosting voltage and capacitance output. Therefore, this work will inspire the new design of mesoporous conducting polymer materials toward high-performance microscale supercapacitive devices.

Self-assembly of biomimetic light-harvesting complexes capable of hydrogen evolution

Biomimetics provides us a new perspective to understand complex biological process and strategy to fabricate functional materials. However,a great challenge still remains to design and fabricate biomimetic materials using a facile but effective method. Here, we develop a biomimetic light harvesting architecture based on one-step co-assembly of amphiphilic amino acid and porphyrin. Amphiphilic amino acid can self-assemble into nanofibers via π-stacking and hydrogen binding interactions. Negatively charged porphyrin adsorbs on the surface of the assembled nanofibers through electrostatic force, and the nanofibers further organize into porous urchin-like microspheres induced presumably by hydrophobic interaction. The assembled amphiphilic amino acid nanofibers work as a template to tune the organization of porphyrin with an architecture principle analogous to natural light harvesting complex. The co-assembled microspheres exhibit enhanced light capture due to the light reflection in the porous structure. Reaction center(platinum nanoparticles) can be effectively coupled with the light harvesting microspheres via photoreduction. After visible light illumination, hydrogen evolution occurs on the hybrid microspheres.

Tunable Multicolor Circularly Polarized Luminescence via Co-assembly of One Chiral Electron Acceptor with Various Donors

Chiroptical materials with multicolor and signinvertible circularly polarized luminescence(CPL)are important for advanced optical devices and display technologies.Here,a general strategy is developed to generate CPL with highly tunable emission bands and handedness through charge-transfer(CT)complexation and co-assembly of one chiral non-emissive tetranitrofluorene-based acceptor with various achiral purple to blue emissive donors.The resulting assemblies exhibit intense CPL with a rich array of colors(519-668 nm)and prominent dissymmetry factors(|glum|)in the range of 10^(-3)-10^(-2).Notably,the CPL sign can be readily inverted by slightly changing the molecular structure of achiral donors.Single-crystal analysis reveals that the donor and acceptor molecules are alternately and asymmetrically packed in a lamellar fashion through CT interactions,leading to efficient transfer of chirality.Furthermore,the refined packing is mediated by the intensity and manner of CT interactions,rendering an inversion of chirality.The chiral co-assembly not only occurs for planar achiral donor molecules,but is also accessible to nonplanar conjugated molecules such as[4]helicene derivatives.Thus,the CPL feature of the resulting products can be easily and broadly manipulated,aiming at advanced chiroptical systems.

Memorable full-color circularly polarized luminescence from chiral co-assembled polymer films enabled by multipath transfer

The modulation of circularly polarized luminescence(CPL)is of importance for display and asymmetric chemical synthesis.However,the underlying mechanism of CPL transfer remains rarely studied.Herein,we rationally design a multipath transfer system including multistep chirality transfer(MCT)and sequential fluorescence resonance energy transfer(SFRET),and we are the first to fabricate memorable full-color CPL-active films with a high dissymmetry factor(~10^(-2)).Specifically,(P/M)-helical nanofibers are constructed by co-assembly between an achiral polymer,poly(9,9-di-n-octylfluorene)(PF8),and easy-to-remove R/S-limonene.When matching achiral emitters are added,the PF8 mediates MCT and simultaneously triggers the SFRET process.Furthermore,full-color CPL memory is realized after removing the chiral source.Molecular simulation and structure analysis indicate that the robust helical superstructure of PF8 provides chiral sites to accommodate emitters,which is essential for CPL transfer and memory.This work provides a novel strategy for constructing CPL-active materials in an aggregated state and insights into CPL transfer and memory.

Self-oxygenated co-assembled biomimetic nanoplatform for enhanced photodynamic therapy in hypoxic tumor

Photodynamic therapy(PDT)has shown great application potential in cancer treatment and the important manifestation of PDT in the inhibition of tumors is the activation of immunogenic cell death(ICD)effects.However,the strategy is limited in the innate hypoxic tumor microenvironment.There are two key elements for the realization of enhanced PDT:specific cellular uptake and release of the photosensitizer in the tumor,and a sufficient amount of oxygen to ensure photodynamic efficiency.Herein,self-oxygenated biomimetic nanoparticles(CS@M NPs)co-assembled by photosensitizer prodrug(Ce6-S-S-LA)and squalene(SQ)were engineered.In the treatment of triple negative breast cancer(TNBC),the oxygen carried by SQ can be converted to reactive oxygen species(ROS).Meanwhile,glutathione(GSH)consumption during transformation from Ce6-S-S-LA to chlorin e6(Ce6)avoided the depletion of ROS.The co-assembled(CS NPs)were encapsulated by homologous tumor cell membrane to improve the tumor targeting.The results showed that the ICD effect of CS@M NPs was confirmed by the significant release of calreticulin(CRT)and high mobility group protein B1(HMGB1),and it significantly activated the immune system by inhibiting the hypoxia inducible factor-1alpha(HIF-1α)-CD39-CD73-adenosine a2a receptor(A2AR)pathway,which not only promoted the maturation of dendritic cells(DC)and the presentation of tumor specific antigens,but also induced effective immune infiltration of tumors.Overall,the integrated nanoplatform implements the concept of multiple advantages of tumor targeting,reactive drug release,and synergistic photodynamic therapy-immunotherapy,which can achieve nearly 90%tumor suppression rate in orthotopic TNBC models.

Site-selection and recognition of aromatic carboxylic acid in response to coronene and pyridine derivative

The self-assembled structures of H_(3) BDA molecule with multiple meta-dicarboxylic groups and their stim-ulus responses to the guest molecules(COR and T4PT)are thoroughly investigated by scanning tunneling microscopy(STM).STM observations display that two kinds of nanostructures are fabricated by H3 BDA molecules through intermolecular hydrogen bonds,in which a linear structure is formed at a higher con-centration and a flower-like structure is obtained at a lower concentration.After the addition of COR and T4PT,H_(3) BDA appears different responsiveness resulting in different co-assembled structures,respectively.The linear structure is regulated into a flower-like structure by COR and COR molecules are trapped in the cavities.When the pyridine derivative(T4PT)is introduced,a new bicomponent porous structure emerges via the hydrogen bond formed between the carboxyl group and the pyridine.Furthermore,the deposition of additional COR to the H_(3) BDA/T4PT system results in the breakdown of the porous structure and the generation of H_(3) BDA/COR host-guest system.Combined with density functional theory(DFT)calculations and molecular dynamics(MD)simulations,the transformation phenomenon of bi-component nanostruc-ture induced by guest molecules is formulated.The results are expected to understand the modification effect of guest molecules on the host network,which is of great significance for the design and construc-tion of multi-component nanostructures and crystal engineering.

A parathyroid hormone related supramolecular peptide for multi-functionalized osteoregeneration

Supramolecular peptide nanofiber hydrogels are emerging biomaterials for tissue engineering,but it is difficult to fabricate multi-functional systems by simply mixing several short-motif-modified supramolecular peptides because relatively abundant motifs generally hinder nanofiber cross-linking or the formation of long nanofiber.Coupling bioactive factors to the assembling backbone is an ideal strategy to design multi-functional supramolecular peptides in spite of challenging synthesis and purification.Herein,a multi-functional supramolecular peptide,P1R16,is developed by coupling a bioactive factor,parathyroid hormone related peptide 1(PTHrP-1),to the basic supramolecular peptide RADA16-I via solid-phase synthesis.It is found that P1R16 self-assembles into long nanofibers and co-assembles with RADA16-I to form nanofiber hydrogels,thus coupling PTHrP-1 to hydrogel matrix.P1R16 nanofiber retains osteoinductive activity in a dose-dependent manner,and P1R16/RADA16-I nanofiber hydrogels promote osteogenesis,angiogenesis and osteoclastogenesis in vitro and induce multi-functionalized osteoregeneration by intramembranous ossification and bone remodeling in vivo when loaded to collagen(Col)scaffolds.Abundant red blood marrow formation,ideal osteointegration and adapted degradation are observed in the 50%P1R16/Col scaffold group.Therefore,this study provides a promising strategy to develop multi-functional supramolecular peptides and a new method to topically administrate parathyroid hormone or parathyroid hormone related peptides for non-healing bone defects.

Zn/Sr dual ions-collagen co-assembly hydroxyapatite enhances bone regeneration through procedural osteo-immunomodulation and osteogenesis

The immune microenvironment induced by biomaterials played vital roles in bone regeneration.Hydroxyapatite(HA)and its ion-substituted derivates represent a large class of core inorganic materials for bone tissue engineering.Although ion substitution was proved to be a potent way to grant HA more biological functions,few studies focused on the immunomodulatory properties of ion-doped HA.Herein,to explore the potential osteoimmunomodulatory effects of ion-doped HA,zinc and strontium co-assembled into HA through a collagen template biomimetic way(ZnSr-Col-HA)was successfully achieved.It was found that ZnSr-Col-HA could induce a favorable osteo-immune microenvironment by stimulating macrophages.Furthermore,ZnSr-Col-HA demonstrated a procedural promoting effect on osteogenic differentiation of bone marrow mesenchymal stem cells(BMSCs)in vitro.Specifically,the osteo-immune microenvironment acted as a dominant factor in promoting osteogenic gene expressions at the early stage through OSM signal pathway.Whereas the direct stimulating effects on BMSCs by Zn^(2+)/Sr^(2+) were more effectively at the later stage with Nfatc1/Maf and Wnt signals activated.In vivo study confirmed strong promoting effects of ZnSr-Col-HA on critical-sized cranial defect repair.The current study indicated that such a combined biomaterial design philosophy of dual ion-doping and biomimetic molecular co-assembly to endow HA applicable osteoimmunomodulatory characteristics might bring up a new cutting-edge concept for bone regeneration study.

Photoresponsive Supramolecular Hydrogel Co-assembled from Fmoc-Phe-OH and 4,4′-Azopyridine for Controllable Dye Release

Photoresponsive hydrogels have been attractive because they can provide precise spatial and temporal control for molecule release, whereas the conventional preparation of photoresponsive hydrogels generally involves complex chemical synthesis steps or specific conditions which limits their practical applications. Herein, a new photoresponsive hydrogel is facilely prepared via co-assembly of two simple molecules, Fmoc-Phe-OH and Azp, without chemical synthesis. The co-assembly mechanism, morphology, and photoresponsiveness of(Fmoc-Phe-OH)-Azp hydrogel are investigated by circular dichroism(CD), ultraviolet-visible(UV-Vis), fluorescence, ~1 H nuclear magnetic resonance(~1 H-NMR), attenuated total internal reflection Fourier transform Infrared(ATR-FTIR) spectroscopy, and scanning electron microscopy(SEM). Furthermore, the enhanced release of encapsulated sulforhodamine B(SRB) dye molecules can be achieved via UV light irradiation. The enhanced dye release amount can be controlled by manipulating photoirradiation time. This study provides a facile way to prepare photoresponsive hydrogel which holds great potential for controllable drug release.

SIMULATION STUDY OF CO-ASSEMBLY OF ABC TRIBLOCK COPOLYMER/NANOPARTICLE INTO MULTICOMPARTMENT HYBRIDS IN SELECTIVE SOLVENT

Co-assembly of ABC linear triblock copolymer/nanoparticle into bump-surface multicompartment hybrids in selective solvent was studied through self-consistent field theory （SCFT） simulation. Results from three-dimensional SCFT simulation showed that the hybrid morphology depended on the length and number of grafted chains, whereas the number and shape of bumps relied on nanoparticle size. Moreover, the simulation results showed that the length and number of grafted chains had equivalent effect on hybrid morphology. Calculated results indicated that entropy was a more important factor than enthalpy in the co-assembly.

Luminous composite ultrathin films of CdTe quantum dots/silk fibroin co-assembled with layered doubled hydroxide:Enhanced photoluminescence and biosensor application

Quantum dots(QDs)luminescent films are extensively applied to optoelectronics and optical devices.However,QDs aggregation results in the quenching of their fluorescence property which limits their practical applications to a greater extent.In order to resolve this issue,3-mercaptopropionic acid(3-MPA)functionalized Cadmium Tellurium(CdTe)QDs were stabilized by silk fibroin(SB)and coassembled with layered doubled hydroxide(LDH)to form(QDs@SF/LDH)_(n)ultrathin films(UTFs)via the layer-by-layer(LBL)technique.UV-Vis absorption and fluorescence spectroscopy showed a stepwise and normal growth of the films upon increasing the number of deposition cycles.XRD and AFM studies confirmed the formation of a periodic layered structure and regular surface morphology of the thin films.As compared to(CdTe QDs/LDH)_(n)UTFs,the(CdTe QDs@SF/LDH)_(n)UTFs displayed fluorescence enhancement and longer fluorescent lifetime,both in solid states and aqueous solutions.Furthermore compared with the solution state,the fluorescence enhancement of SF-RC and SF-β are,respectively,7 times and 17 times in the(CdTe QDs@SF/LDH)_(n)UTFs,indicating that the LDH nanosheets favor the fluorescence enhancement effect on the CdTe QDs@SF.The fabricated materials displayed fluorescence response to a biological molecule such as immune globulin,lgG.Thus,the(CdTe QDs@SF/LDH)_(n)UTFs has a potential to be used as biosensor.

Supramolecular co-assembly of self-adjuvanting nanofibrious peptide hydrogel enhances cancer vaccination by activating MyD88-dependent NF-κB signaling pathway without inflammation

Peptide vaccine targeting tumor-specific antigens is a promising cancer treatment regimen.However,peptide vaccines are commonly low-immunogenic,leading to suboptimal antitumor T-cell responses.Current peptide vaccination approaches are challenged by the variability of peptide physicochemical characters and vaccine formulations,flexibility,and the broad feasibility.Here,the supramolecular co-assembly of antigen epitope-conjugated peptides(ECPs)targeting CD8 or CD4 T-cell receptors was used to engineer a nanofibrious hydrogel vaccine platform.This approach provided precise and tunable loading of peptide antigens in nanofibers,which notably increased the antigen uptake,cross-presentation,and activation of dendritic cells(DCs).Immunization in mice indicated that the co-assembled peptide hydrogel did not induce local inflammation responses and elicited significantly promoted T-cell immunity by activating the MyD88-dependent NF-κB signaling pathway in DCs.Vaccination of mice using co-assembled peptide vaccine stimulated both enhanced CD8 and CD4 T cells against EG.7-OVA tumors without additional immunoadjuvants or delivery systems,and resulted in a more remarkable cancer immunotherapy efficacy,compared with free peptide vaccine or aluminum-adjuvanted peptide formulation.Altogether,peptide co-assembly demonstrated by three independent pairs of ECPs is a facile,customizable,and chemically defined approach for co-delivering peptide antigens in self-adjuvanting hydrogel vaccines that could induce stronger anticancer T-cell responses.

Polymer-virus core-shell structures prepared via co-assembly and template synthesis methods

Bionanoparticles(BNPs),consisting of virus and virus-like assemblies,have attracted much attention in the biomedical field for their applications such as imaging and targeted drug delivery,owing to their well-defined structures and well-controlled chemistries.BNPs-based core-shell structures provide a unique system for the investigation of biological interactions such as protein-protein and protein-carbohydrate interactions.However,it is still a challenge to prepare the BNPs-based core-shell structures.Herein,we describe(i) co-assembly method and(ii) template synthesis method in the development of polymer-BNPs core-shell structures.These two methods can be divided into three different systems.In system A,different polymers including poly(2-vinylpyridine)(P2VP),poly(4-vinylpyridine)(P4VP) and poly(ε-caprolactone)-block-poly(2-vinylpyridine)(PCL-b-P2VP) can form a raspberry-like structure with BNPs.In system B,polystyrene(PS) spheres end capped with free amine and BNPs can form a core-shell structure.In System C,layer-by-layer(LBL) method is used to prepare positive charged PS particles,which can be used as a template to form the core-shell structures with BNPs.These two methods may open a new way for preparing novel protein-based functional materials for potential applications in the biomedical field.

A polymeric co-assembly of subunit vaccine with polyoxometalates induces enhanced immune responses

Long-lasting protective immune responses are expected following vaccination.However,most vaccines alone are inability to evoke an efficient protection.The combinatory administration of adjuvants with vaccines is critical for generating the enhanced immune responses.Herein,with biocompatible poly(4-vinylpyridine)(P4VP)as template,2.5 nm iron/molybdenum oxide cluster,{Mo_(72)Fe_(30)},is applied as an adjuvant to co-assemble with antigens of Mycobacterium bovis via hydrogen bonding at molecular scale.Molecular scale integration of the antigens and{Mo_(72)Fe_(30)}and their full exposure to body fluid media contribute to the augmentation of both humoral and cellular immune responses of the vaccines after inoculation in mice.Anti-inflammatory factor IL-10 gradually increases after 2 weeks followed by a final back to normal level by the 5th week.The balance between proinflammatory cytokines and anti-inflammatory factors suggests that immune system can be activated in the early stage of infection by the antigens carried by the supra-particles and secrete acute inflammatory factors for host defense and anti-inflammatory factors for immune protection.

Mechanical Reinforcement of Molecular Hydrogel by Co-assembly of Short Peptide-based Gelators with Different Aromatic Capping Groups

We demonstrated in this study that mixing two short peptide-based gelators with different aromatic capping groups would result in molecular hydrogels with enhanced mechanical property.We selected gelators of PTZ-GFFY and Nap-GFFY for the study.Both PTZ-GFFY and Nap-GFFY could independently form molecular hydrogel by a heating-cooling process.However,the mechanical property of gels of PTZ-GFFY and Nap-GFFY was relatively weak with storage moduli(G')of about 500 and 150 Pa,respectively.A two-component gel of PTZ-GFFY and Nap-GFFY could also form by a heating-cooling process.Surprisingly,the G'value of the two-component gel was about 5000 Pa,which was at least ten times bigger than that of each single-component gel.This is a novel and simple strategy to improve the mechanical property of molecular hydrogels.

KLVFF peptide functionalized nanoparticles capture Aβ_(42) by co-assembly for decreasing cytotoxicity

The accumulation of extracellular senile plagues of amyloid β-proteins(Aβ) in the patient brain is one of most important facture in Alzheimer’s disease(AD), which is resulted from abnormal self-assembly of Aβpeptide. Nanoparticles show greatly potential for therapeutic studies due to the sophisticated surface which can mimic the functionality of protein. Herein, we design the bis(pyrene)-Lys-Leu-Val-Phe-PheGly-poly ethylene glycol(BP-KLVFFGPEG) based nanoparticles, which capture Aβthrough recognition and co-assembly of KLVFF. The resulting co-assemblies show different morphologies, such as large aggregates, nanoparticles and nanofibers corresponding to the different length chain of PEG, leading to different capture efficiencies. The co-assembly strategy shows a decrease of cytotoxicity, potentially for AD treatment.

Influence of Mixed Common Solvent on the Co-assembled Morphology of PS-b-PEO and CdS Quantum Dots

The hybrid structures of polystyrene-b-poly（ethylene oxide） （PS-b-PEO） block copolymer and inorganic nanoparticles with good stability and biocompatibility have potential applications in drug delivery and bioimaging. Spherical co-assemblies of PS120-b-PEO318 and oleylamine-capped CdS quantum dots （QDs） are produced successfully in this work by adding water to a mixed common solvent, such as N,N-dimethylmethanamide （DMF）/chloroform, DMF/tetrahydrofuran （THF）, or DMF/toluene. The energy dispersive X-ray （EDX） spectrum indicates that QDs are located at the interface between the core and shell of the spherical co-assemblies. The co-assembly process during water addition is traced by transmission electron microscopy （TEM） and turbidity measurement. Spherical co-assemblies are formed through budding from bilayers of the block copolymer and QDs. The morphology of the co-assemblies is related to the miscibility of the QD-dispersing solvents with water and the morphology changes from a spherical to a vesicle-like structure with DMF/toluene. Increasing THF content in the mixed solvent causes morphological transitions from spherical co-assemblies to multi-branched cylinders and micelles where QDs are located in the central core. Increasing chloroform content yields vesicle-like structures with protruding rods on the surface. The mechanism of the morphological transitions is also discussed in detail.

The self-assembly and structural regulation of a hydrogen-bonded dimeric building block formed by two N-H…O hydrogen bonds on HOPG

The self-assembled behavior of an unsymmetric molecule(BCDTDA)with one imidazole group as center and benzoic acid group as functional group is studied,and the regulatory behaviors of coronene(COR)and three bipyridine derivatives(named BP,PEBP-C4 and PEBP-C8)on BCDTDA self-assembly structures are also investigated.Based on highly oriented pyrolytic graphite(HOPG)substrate,scanning tunneling microscopy(STM)is used to observe the variation of assembled behaviors at the solid-liquid interface.Because of the concentration effect,BCDTDA molecules can assemble into grids and Kagomés structures in the form of N–H…O hydrogen bonded dimers.BCDTDA molecules still maintain dimeric structures in the regulation of COR and BP molecules to BCDTDA self-assembly.However,PEBP-C4 and PEBP-C8 destroy the structure of the dimers,and form a variety of co-assembled structures with BCDTDA.Different guest molecules coordinate the host molecules differently,which makes the experiment more meaningful.Combined with density functional theory(DFT)calculation,the discovery of molecular interactions provides a promising strategy for the construction of functional nanostructures and devices.

Probing the electrostatic aggregation of nanoparticles with oppositely charged molecular ions

The co-assembly of charged nanoparticles with oppositely charged molecular ions has emerged as a promising technique in the fabrication of nanoparticle superstructures.However,the underlying mechanism behind these molecular ions in mediating the repulsion between these charged nanoparticles remains elusive.Herein,coarsegrained molecular dynamics simulations are used to elucidate the effects of valency,shape,and size of molecular anions on their co-assembly with gold nanoparticles coated with positively charged ligands.The findings suggest that the valency,shape,and size of molecular anions significantly influence the repulsion and aggregating dynamics among these positively charged nanoparticles.Moreover,the free energy calculations reveal that ring-shaped molecular anions with higher valences and larger sizes are more effective at reducing the repulsion between these gold nanoparticles and thus enhance the stability of the aggregate.This study contributes to a better understanding of the critical roles of valence,shape,and size of ions in mediating the electrostatic co-assembly of nanoparticles with oppositely charged ions,and it also guides the future design of DNA templates and DNA origami in co-assembly with oppositely charged nanoparticles.