Surface tailoring of nanoparticles via mixed-charge monolayers and their biomedical applications

2D MXene-Based Biosensing: A Review

MXene emerged as decent 2D material and has been exploited for numerous applications in the last decade. The remunerations of the ideal metallic conductivity, optical absorbance, mechanical stability, higher heterogeneous electron transfer rate, and good redox capability have made MXene a potential candidate for biosensing applications. The hydrophilic nature, biocompatibility, antifouling, and anti-toxicity properties have opened avenues for MXene to perform in vitro and in vivo analysis. In this review, the concept, operating principle, detailed mechanism, and characteristic properties are comprehensively assessed and compiled along with breakthroughs in MXene fabrication and conjugation strategies for the development of unique electrochemical and optical biosensors. Further, the current challenges are summarized and suggested future aspects. This review article is believed to shed some light on the development of MXene for biosensing and will open new opportunities for the future advanced translational application of MXene bioassays.

Self-Complementary Zwitterionic Peptides Direct Nanoparticle Assembly and Enable Enzymatic Selection of Endocytic Pathways

Supramolecular self-assembly in biological systems holds promise to convert and amplify disease-specific signals to physical or mechanical signals that can direct cell fate. However, it remains challenging to design physiologically stable self-assembling systems that demonstrate tunable and predictable behavior. Here, the use of zwitterionic tetrapeptide modalities to direct nanoparticle assembly under physiological conditions is reported. The self-assembly of gold nanoparticles can be activated by enzymatic unveiling of surface-bound zwitterionic tetrapeptides through matrix metalloprotease-9 (MMP-9), which is overexpressed by cancer cells. This robust nanoparticle assembly is achieved by multivalent, self-complementary interactions of the zwitterionic tetrapeptides. In cancer cells that overexpress MMP-9, the nanoparticle assembly process occurs near the cell membrane and causes size-induced selection of cellular uptake mechanism, resulting in diminished cell growth. The enzyme responsiveness, and therefore, indirectly, the uptake route of the system can be programmed by customizing the peptide sequence: a simple inversion of the two amino acids at the cleavage site completely inactivates the enzyme responsiveness, self-assembly, and consequently changes the endocytic pathway. This robust self-complementary, zwitterionic peptide design demonstrates the use of enzyme-activated electrostatic side-chain patterns as powerful and customizable peptide modalities to program nanoparticle self-assembly and alter cellular response in biological context.

Mixed-Charge Nanocarriers Allow for Selective Targeting of Mitochondria by Otherwise Nonselective Dyes

Targeted delivery of molecular cargos to specific organelles is of paramount importance for developing precise and effective therapeutics and imaging probes. This work describes a disulfide-based delivery method in which mixed-charged nanoparticles traveling through the endolysosomal tract deliver noncovalently bound dye molecules selectively into mitochondria. This system comprises three elements: (1) The nanoparticles deliver their payloads by a kiss-and-go mechanism - that is, they drop off their dye cargos proximate to mitochondria but do not localize therein; (2) the dye molecules are by themselves nonspecific to any cellular structures but become so with the help of mixed-charge nanocarriers; and (3) the dye is engineered in such a way as to remain in mitochondria for a long time, up to days, allowing for observing dynamic remodeling of mitochondrial networks and long-term tracking of mitochondria even in dividing cells. The selectivity of delivery and long-lasting staining derive from the ability to engineer charge-imbalanced, mixed [+/-] on-particle monolayers and from the structural features of the cargo. Regarding the former, the balance of [+] and [-] ligands can be adjusted to limit cytotoxicity and control the number of dye molecules adsorbed onto the particles' surfaces. Regarding the latter, comparative studies with multiple dye derivatives we synthesized rationalize the importance of polar groups, long alkyl chains, and disulfide moieties in the assembly of fluorescent nanoconstructs and long-lasting staining of mitochondria. Overall, this strategy could be useful for delivering hydrophilic and/or anionic small-molecule drugs difficult to target to mitochondria by classical approaches.

Metallic-Nanoparticle-Based Sensing: Utilization of Mixed-Ligand Monolayers

The last two decades have seen great advancements in fundamental understanding and applications of metallic nanoparticles stabilized by mixed-ligand monolayers. Identifying and controlling the organization of multiple ligands in the nanoparticle monolayer has been studied, and its effect on particle properties has been examined. Mixed-ligand protected particles have shown advantages over monoligand protected particles in fields such as catalysis, self-assembly, imaging, and drug delivery. In this Review, the use of mixed-ligand monolayer protected nanoparticles for sensing applications will be examined. This is the first time this subject is examined as a whole. Mixed-ligand nanoparticle-based sensors are revealed to be divided into four groups, each of which will be discussed. The first group consists of ligands that work cooperatively to improve the sensors' properties. In the second group, multiple ligands are utilized for sensing multiple analytes. The third group combines ligands used for analyte recognition and signal production. In the final group, a sensitive, but unstable, functional ligand is combined with a stabilizing ligand. The Review will conclude by discussing future challenges and potential research directions for this promising subject.

A pH-Sensitive Self-Assembled and Carrier-Free Nanoparticle Based on Charge Reversal for Enhanced Synergetic Chemo-Phototherapy

To overcome biological barriers for nanoparticles (NPs) efficaciously accumulated at tumor sites, as well as enhancing the performance of drug delivery systems, a carrier-free nanoparticle based on charge reversal is designed for improved synergetic chemo-phototherapy for cancer treatment. In this system, doxorubicin (Dox) and zinc phthalocyanine (ZnPc) are self-assembled through noncovalent interactions (π-π stacking, hydrophobic forces) to avoid the possible toxicity of excipient, complex chemical conjugations and batch-to-batch variation. A trace amount of poly(2-(di-methylamino) ethylmethacrylate)- poly[(R)-3-hydroxybutyrate]- poly(2-(dimethylamino) ethylmethacrylate (PDMAEMA-PHB-PDMAEMA) is modified on the surface of Dox-ZnPc to construct the novel nanoparticles, namely DZP, with long-term stability, and with a dual-drug load content of up to ≈90％. The drug delivery system (DDS) can effectively decrease its toxicity among physical circulation and increase the accumulation at the tumor site. Moreover, the developed DZP nanoparticles show excellent photo-chemotherapy, photoacoustic (PA) and fluorescence (FL) imaging characteristics for multimodal imaging-guided synergetic therapy.

iRGD Peptide as a Tumor-Penetrating Enhancer for Tumor-Targeted Drug Delivery

The unique structure and physiology of a tumor microenvironment impede intra-tumoral penetration of chemotherapeutic agents. A novel iRGD peptide that exploits the tumor microenvironment can activate integrin-dependent binding to tumor vasculatures and neuropilin-1 (NRP-1)-dependent transport to tumor tissues. Recent studies have focused on its dual-targeting ability to achieve enhanced penetration of chemotherapeutics for the efficient eradication of cancer cells. Both the covalent conjugation and the co-administration of iRGD with chemotherapeutic agents and engineered delivery vehicles have been explored. Interestingly, the iRGD-mediated drug delivery also enhances penetration through the blood-brain barrier (BBB). Recent studies have shown its synergistic effect with BBB disruptive techniques. The efficacy of immunotherapy involving immune checkpoint blockades has also been amplified by using iRGD as a targeting moiety. In this review, we presented the recent advances in iRGD technology, focusing on cancer treatment modalities, including the current clinical trials using iRGD. The iRGD-mediated nano-carrier system could serve as a promising strategy in drug delivery to the deeper tumor regions, and be combined with various therapeutic interventions due to its novel targeting ability.

Introduction of lactobionic acid ligand into mixed-charge nanoparticles to realize in situ triggered active targeting to hepatoma cells

To overcome the dilemma between passive tissue targeting and active cell targeting, nanomaterials are often required to exhibit the transition from ‘stealth’ to ‘active targetable’ in response to the pathological microenvironment. Here, we introduced a ternary surface modification method that incorporating active targeting ligand lactobionic acid with pH-sensitive mixed-charge surface. The resulted mixed-charge gold nanoparticles (LA@MC-GNPs) showed resistance to non-specific adsorption of proteins and uptake by HepG2 cells at normal tissue pH 7.4, while they underwent pH-sensitive aggregation and recovered active targeting capability at tumor acidic pH 6.5. The ternary surface modification method provided a simplest strategy to solve the dilemma between passive and active targeting of nanomedicine.

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Gold nanoparticles (GNPs) were ternary modified with lactobionic acid ligand and mixed-charge ligands.

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The resulted GNPs showed pH-sensitive aggregation in response to tumor acidic pH.

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Resistance to non-specific protein adsorption and cell uptake of the ternary modified GNPs was observed at pH 7.4.

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Cell uptake was greatly enhanced because of the recalled active targeting ability at pH 6.5.

Fluorescent Carbon Dots from Nerium oleander: Effects of Physical Conditions and the Extract Types

In this original research, the synthesis of carbon nanodots (CDs) from two different solvent extracts of Nerium oleander by the thermal method was investigated under various physical conditions such as pH, reaction temperature, ionic strength, and surface passivation agent (polyethylene glycol, PEG) presence in the reaction media. The effects of extract types and physical conditions on CDs formation were characterized by UV-Visible spectrophotometry, fluorescence spectrophotometry, Fourier transform infrared spectroscopy and dynamic light scattering analysis. Fluorescent CDs were obtained from PEG included reaction media. Additionally, the enhanced fluorescence intensity correlated with ascending reaction temperature was reported. The hydrodynamic particle size of CDs in aqueous solution was determined between ~1 and 235 nm with negative surface potential in the range of -6 mV and -28 mV. Moreover, CDs synthesized from aqueous extract mostly resulted in smaller size than that of ethanol extract based ones. The impact of surface passivation with PEG on the fluorescence feature of CDs was verified. For the relevant extracts of Oleander, CDs synthesized from PEG included formulations at pH 5 and NaCl free reaction media found as better alternatives than CDs synthesized under other conditions taking account their effect on fluorescence feature, hydrodynamic size and etc. Graphical Abstract.

mRNA Guided Intracellular Self-Assembly of DNA-Gold Nanoparticle Conjugates as a Precise Trigger to Up-Regulate Cell Apoptosis and Activate Photothermal Therapy

The size of nanoparticles was generally accepted to have a close relationship with the penetration and retention properties among tumor sites, which is one of the most significant issues during nanomedicine delivery. Despite the outstanding stealth property when circulating and the penetration ability in tumor tissue, small nanoparticles still have the problem of inadequate retention time. Taking advantage of the precise self-assembly of DNA-nanoparticle conjugates, we developed an intracellular assembly system to realize the change of nanoparticle size from small to large as well as activation of therapeutic function inside cancer cells. A duplex sequence of cancer-cell-specific mRNA, survivin, was selected to hybridize with complementary sequence of gold nanoparticle-DNA (AuNP-DNA) conjugates in cancer cell cytoplasm, resulting in the specific and precise formation of intracellular assemblies. Enhanced retention behavior of AuNPs inside cancer cells was shown to be achieved because of the increased nanoparticle size. Meanwhile, an up-regulation effect of cell apoptosis and an activated photothermal therapy function were also created by the formation of AuNP aggregations, and eventually contributed to a high rate of cancer cells death up to 93.33%. In contrast, it exhibited almost no toxicity toward normal cells because of the absence of survivin-induced assembly. Therefore, this mRNA guided intracellular assembly system exhibited its potential as a new precise cancer therapy strategy, and also broadened the application field of DNA-conjugated nanoparticle assembly.

Photosensitizer-Loaded Multifunctional Chitosan Nanoparticles for Simultaneous in Situ Imaging, Highly Efficient Bacterial Biofilm Eradication, and Tumor Ablation

In recent decades, bacterial and viral infections and chronic inflammatory response have emerged as important causes of cancer. Also, infections remain a significant cause of morbidity and mortality in cancer patients. In this work, carboxymethyl chitosan nanoparticles (CMC NPs) were synthesized in a facile and green way and further combined with ammonium methylbenzene blue (MB) as a cross-linking agent as well as a fluorescent molecule and a photosensitizer for self-imaging photodynamic therapy (PDT). The obtained CMC-MB NPs exhibited an apparent pH-responsive release behavior of MB, which was released for a prolonged period in a simulated physiological environment (pH 7.4) for more than 15 days and the time reduced to only 3.5 h in acidic conditions (pH 5.5). When irradiated by a 650 nm laser at 202 mW/cm² for 5 min, the CMC-MB NPs showed efficient bactericidal and biofilm eradication properties as well as suppression of tumor cell growth in a similar acidified microenvironment. Furthermore, in an in vivo rabbit wound bacterial infection model, the rapid sterilization of CMC-MB NPs played a crucial role in bacterial infections, inflammation inhibition, and wound healing. As a PDT treatment against cancer, the CMC-MB NPs also exhibited an efficient antitumor therapeutic effect in a subcutaneous tumor mice model.

Endogenous pH-responsive nanoparticles with programmable size changes for targeted tumor therapy and imaging applications

Nanotechnology-based antitumor drug delivery systems, known as nanocarriers, have demonstrated their efficacy in recent years. Typically, the size of the nanocarriers is around 100 nm. It is imperative to achieve an optimum size of these nanocarriers which must be designed uniquely for each type of delivery process. For pH-responsive nanocarriers with programmable size, changes in pH (~6.5 for tumor tissue, ~5.5 for endosomes, and ~5.0 for lysosomes) may serve as an endogenous stimulus improving the safety and therapeutic efficacy of antitumor drugs. This review focuses on current advanced pH-responsive nanocarriers with programmable size changes for anticancer drug delivery. In particular, pH-responsive mechanisms for nanocarrier retention at tumor sites, size reduction for penetrating into tumor parenchyma, escaping from endo/lysosomes, and swelling or disassembly for drug release will be highlighted. Additional trends and challenges of employing these nanocarriers in future clinical applications are also addressed.

Preparation of Vascular Endothelial Cadherin Loaded-Amphoteric Copolymer Decorated Coronary Stents for Anticoagulation and Endothelialization

A new strategy for preparation of blood-contact materials, with their short-term anticoagulation depending on zwitterionic structure and long-term hemocompatibility based on endothelialization, was proposed, performed, and proved. The copolymer made of sulfonamide zwitterionic and acrylic acid was designed and synthesized, and grafted to the surface of the bare metal coronary stent. Then, the vascular endothelial cadherin (VE-Cad), one of the specific antibodies of endothelial progenitor cells (EPCs), was fixed onto the copolymer chain. Finally, it is proved by in vitro blood tests that the coronary stent decorated with VE-Cad loaded-amphoteric copolymer displayed good platelet anti-adhesion characteristic. This anti-adhesion characteristic was attributed to the zwitterionic structure and the biofunctionality of specifically capturing EPCs confirmed by the results that the antibody-decorated coronary stent was trapped with EPCs. Finally, the in vivo implantation experiments of the antibody-decorated coronary stent in rabbit for 4 weeks were carried out. Results indicated that the endothelium and smooth surface of the antibody-loaded stent was found to be due to the covered effect of EPCs, without obvious intimal hyperplasia. The strategy we proposed has great potential in the design and preparation of blood-contact biomedical materials and devices.

Diverse Applications of Nanomedicine

The design and use of materials in the nanoscale size range for addressing medical and health-related issues continues to receive increasing interest. Research in nanomedicine spans a multitude of areas, including drug delivery, vaccine development, antibacterial, diagnosis and imaging tools, wearable devices, implants, high-throughput screening platforms, etc. using biological, nonbiological, biomimetic, or hybrid materials. Many of these developments are starting to be translated into viable clinical products. Here, we provide an overview of recent developments in nanomedicine and highlight the current challenges and upcoming opportunities for the field and translation to the clinic.

Molecular Understanding on the Underwater Oleophobicity of Self-Assembled Monolayers: Zwitterionic versus Nonionic

Molecular dynamics simulations are conducted to investigate the underwater oleophobicity of self-assembled monolayers (SAMs) with different head groups. Simulation results show that the order of underwater oleophobicity of SAMs is methyl < amide < oligo(ethylene glycol) (OEG) < ethanolamine (ETA) < hydroxyl < mixed-charged zwitterionic. The underwater-oil contact angles (OCAs) are <133° for all nonionic hydrophilic SAMs, while the mixed-charged zwitterionic SAMs are underwater superoleophobic (OCA can reach 180°). It appears that surfaces with stronger underwater oleophobicity have better antifouling performance. Further study on the effect of different alkyl ammonium ions on mixed-charged SAMs reveals that the underwater OCAs are >143.6° for all SAMs; mixed-charged SAMs containing primary alkyl ammonium ion are likely to possess the best underwater oleophobicity for its strong hydration capacity. It seems that alkyl sulfonate anion (SO₃^-) is more hydrophilic than alkyl trimethylammonium ion (NC₃⁺) for the hydrophobic methyl groups on nitrogen atoms and that the hydration of SO₃^- in mixed-charged SAMs can be seriously blocked by NC₃⁺. The monomer of SO₃^- should be slightly longer than that of NC₃⁺ to obtain better underwater oleophobicity in NC₃⁺-/SO₃^--SAMs. In addition, the underwater oleophobicity of SAMs might become worse at low grafting densities. This work systematically proves that a zwitterionic surface is more underwater oleophobic than a nonionic surface. These results will help for the design and development of superoleophobic surfaces.

A Multifunctional Platform for Tumor Angiogenesis-Targeted Chemo-Thermal Therapy Using Polydopamine-Coated Gold Nanorods

Image-guided combined chemo-thermal therapy assists in optimizing treatment time, enhancing therapeutic efficiency, and circumventing side effects. In the present study, we developed a chemo-photothermal theranostic platform based on polydopamine (PDA)-coated gold nanorods (GNRs). The PDA coating was thin; however, it significantly suppressed the cytotoxicity of the cetyltrimethylammonium bromide template and allowed high cisplatin loading efficiency, arginine-glycine-aspartic acid (RGD) peptide (c(RGDyC)) conjugation, and chelator-free iodine-125 labeling (RGD-¹²⁵IPt-PDA@GNRs). While loaded cisplatin was released in a pH-sensitive manner, labeled ¹²⁵I was outstandingly stable under biological conditions. RGD-¹²⁵IPt-PDA@GNRs had a high specificity for αvβ₃ integrin, and consequently, they could selectively accumulate in tumors, as revealed by single photon emission computed tomography/CT imaging, and in target tumor angiogenic vessels, as shown by high-resolution photoacoustic imaging. As RGD-¹²⁵IPt-PDA@GNRs targets tumor angiogenesis, it is a highly potent tumor therapy. Combined chemo-photothermal therapy with probes could thoroughly ablate tumors and inhibit tumor relapse via a synergistic antitumor effect. Our studies demonstrated that RGD-¹²⁵IPt-PDA@GNRs is a robust platform for image-guided, chemo-thermal tumor therapy with outstanding synergistic tumor killing and relapse inhibition effects.

Systems-level thinking for nanoparticle-mediated therapeutic delivery to neurological diseases

Neurological diseases account for 13% of the global burden of disease. As a result, treating these diseases costs $750 billion a year. Nanotechnology, which consists of small (~1-100 nm) but highly tailorable platforms, can provide significant opportunities for improving therapeutic delivery to the brain. Nanoparticles can increase drug solubility, overcome the blood-brain and brain penetration barriers, and provide timed release of a drug at a site of interest. Many researchers have successfully used nanotechnology to overcome individual barriers to therapeutic delivery to the brain, yet no platform has translated into a standard of care for any neurological disease. The challenge in translating nanotechnology platforms into clinical use for patients with neurological disease necessitates a new approach to: (1) collect information from the fields associated with understanding and treating brain diseases and (2) apply that information using scalable technologies in a clinically-relevant way. This approach requires systems-level thinking to integrate an understanding of biological barriers to therapeutic intervention in the brain with the engineering of nanoparticle material properties to overcome those barriers. To demonstrate how a systems perspective can tackle the challenge of treating neurological diseases using nanotechnology, this review will first present physiological barriers to drug delivery in the brain and common neurological disease hallmarks that influence these barriers. We will then analyze the design of nanotechnology platforms in preclinical in vivo efficacy studies for treatment of neurological disease, and map concepts for the interaction of nanoparticle physicochemical properties and pathophysiological hallmarks in the brain. WIREs Nanomed Nanobiotechnol 2017, 9:e1422. doi: 10.1002/wnan.1422 For further resources related to this article, please visit the WIREs website.

Zwitterionic Phosphorylcholine-TPE Conjugate for pH-Responsive Drug Delivery and AIE Active Imaging

Polymeric micelles have emerged as a promising nanoplatform for cancer theranostics. Herein, we developed doxorubicin (DOX) encapsulated pH-responsive polymeric micelles for combined aggregation induced emission (AIE) imaging and chemotherapy. The novel zwitterionic copolymer poly(2-methacryloyloxyethylphosphorylcholine-co-2-(4-formylphenoxy)ethyl methacrylate) (poly(MPC-co-FPEMA)) was synthesized via RAFT polymerization and further converted to PMPC-hyd-TPE after conjugation of tetraphenylethene (TPE, a typical AIE chromophore) via acid-cleavable hydrazone bonds. The AIE activatable copolymer PMPC-hyd-TPE could self-assemble into spherical PC-hyd-TPE micelles, and DOX could be loaded through hydrophobic interactions. The zwitterionic micelles exhibited excellent physiological stability and low protein adsorption due to the stealthy phosphorylcholine (PC) shell. In addition, the cleavage of hydrophobic TPE molecules under acidic conditions could induce swelling of micelles, which was verified by size changes with time at pH 5.0. The in vitro DOX release profile also exhibited accelerated release rate with pH value decreasing from 7.4 to 5.0. Fluorescent microscopy and flow cytometry studies further demonstrated fast internalization and accumulation of drug loaded PC-hyd-TPE-DOX micelles in HepG2 cells, resulting in considerable time/dose-dependent cytotoxicity. Meanwhile, high-quality AIE imaging of PC-hyd-TPE micelles was confirmed in HepG2 cells. Notably, ex vivo imaging study exhibited efficient accumulation and drug release of PC-hyd-TPE-DOX micelles in the tumor tissue. Consequently, the multifunctional micelles with combined nonfouling surface, AIE active imaging, and pH-responsive drug delivery showed great potential as novel nanoplatforms for a new generation of cancer theranostics.

Ligand Layer Engineering To Control Stability and Interfacial Properties of Nanoparticles

The use of mixed ligand layers including poly(ethylene glycol)-based ligands for the functionalization of nanoparticles is a very popular strategy in the context of nanomedicine. However, it is challenging to control the composition of the ligand layer and maintain high colloidal and chemical stability of the conjugates. A high level of control and stability are crucial for reproducibility, upscaling, and safe application. In this study, gold nanoparticles with well-defined mixed ligand layers of α-methoxypoly(ethylene glycol)-ω-(11-mercaptoundecanoate) (PEGMUA) and 11-mercaptoundecanoic acid (MUA) were synthesized and characterized by ATR-FTIR spectroscopy and gel electrophoresis. The colloidal and chemical stability of the conjugates was tested by dynamic light scattering (DLS), small-angle X-ray scattering (SAXS), and UV/vis spectroscopy based experiments, and their interactions with cells were analyzed by elemental analysis. We demonstrate that the alkylene spacer in PEGMUA is the key feature for the controlled synthesis of mixed layer conjugates with very high colloidal and chemical stability and that a controlled synthesis is not possible using regular PEG ligands without the alkylene spacer. With the results of our stability tests, the molecular structure of the ligands can be clearly linked to the colloidal and chemical stabilization. We expect that the underlying design principle can be generalized to improve the level of control in nanoparticle surface chemistry.

Molecular Understanding and Structural-Based Design of Polyacrylamides and Polyacrylates as Antifouling Materials

Design and synthesis of highly bioinert and biocompatible antifouling materials are crucial for a broad range of biomedical and engineering applications. Among antifouling materials, polyacrylamides and polyacrylates have proved so promising because of cheap raw materials, ease of synthesis and applicability, and abundant functional groups. The strong surface hydration and the high surface packing density of polyacrylamides and polyacrylates are considered to be the key contributors to their antifouling property. In this article, we review our studies on the design and synthesis of a series of polyacrylamides and polyacrylates with different molecular structures. These polymers can be fabricated into different architectural forms (brushes, nanoparticles, nanogels, and hydrogels), all of which are highly resistant to the attachment of proteins, cells, and bacteria. We find that small structural changes in the polymers can lead to large enhancement in surface hydration and antifouling performance, both showing a positive correlation. This reveals a general design rule for effective antifouling materials. Furthermore, polyacrylamides and polyacrylates are readily functionalized with other bioactive compounds to achieve different new multifunctionalities.