Self-assembled nanomaterials for biosensing and therapeutics: recent advances and challenges

Shan Huang , Yuexin Song , Zhimei He , Jian-Rong Zhang and Jun-Jie Zhu *
State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, Jiangsu 210023, China. E-mail: jjzhu@nju.edu.cn

First published on 10th March 2021

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1. Introduction

Nanomaterials, defined as materials with nanoscale structures, have been used in different fields including biosensing and biomedicine. These nanostructures have unique properties and may exhibit other chemical, physical or biological properties and functions that conventional size materials do not possess. Particularly, benefiting from the controllable self-assembly behaviour, self-assembled nanomaterials (SANs) exhibit well-defined structures and unique optical properties, providing new materials for biosensing and therapeutics. Some natural units such as subcellular organelles, vesicles produced by cells, can be manufactured into SANs through various assembly processes. Such natural SANs are of great significance to the ordinary processes of life. Inspired by this, artificial SANs have emerged and may become an essential tool for revealing and understanding life activities. For example, some building blocks (e.g., DNA,^1–3 peptides,^4–7 polymers,^8,9 fluorescent organics^10,11 and proteins^12,13) can self-assemble into biofunctional nanostructures suitable for biosensing and bioimaging, which have attracted great interest in recent years.¹⁴ Although great progress has been made, the current SANs still face challenges in dealing with more accurate theranostics of early-stage diseases; therefore, more effort needs to be devoted to this field.

The increasing mature nanoparticle assembly strategies and further exploration of biological functions have promoted the next-generation of SANs. Unlike single-component SANs, nanoassemblies composed of multiple units present unexpected optical features, intracellular stability, and ease of internalization, providing broad application prospects.

To date, some novel strategies have been reported to detect the biological targets in living cells. For instance, gold plasmonic dimers and self-assembled pyramids can accomplish highly sensitive and quantitative detection of targets in complex and dynamic physiological environments.^15,16 The precise control over signal amplification creates unparalleled opportunities for diverse applications. In this regard, some new design methodologies have been proposed to remotely and spatiotemporally manipulate the signal amplification at the region of interest to analyse the targets for accurate diagnosis of early-stage cancers.^17,18 In addition to improved detection sensitivity and designability, SANs are also capable of imaging multiple targets in living cells.¹⁹ Since various biomolecules have characteristic expression profiles at the tumor sites, SAN-based nanoprobes can help us better understand the physiological functions of targets and their regulatory roles during pathological development.²⁰ Typically, some SANs can simultaneously monitor and image multiple intracellular objects to avoid false-positive detection, thus improving the accuracy of cancer diagnosis.²¹ Besides, the precise control of SANs also makes them promising in the biomedical field and the fight against age-related diseases. For instance, plasmonic assembled nanostructures can cause mitochondrial damage and enhance immune response,²² and protein-biomineralized metal sulfide nanoparticles can pave a new avenue for biomedical applications (Fig. 1).²³

This review will introduce various SANs for biosensing and cancer therapy reported in the past five years. The following part will summarize the design concepts of SANs and the corresponding advantages. In the end, we will point out the current challenges of SANs in biosensing/biomedical applications and give an outlook on the future improvement of SANs.

2. Single-component self-assembled nanomaterials

Benefiting from the prosperous development of self-assembly technology, SANs have become one of the vital explorations in nanoscience, which may create promising breakthroughs in several applications. Different types of assembly units (e.g., DNA, peptides, polymers, fluorescent organics, and proteins) are extensively studied in various fields.

2.1. Self-assembled DNA

DNA stores rich genetic information to guide the development and physiological functions of organisms. Therefore, it is described as the ‘blueprint’ of life. The double helix DNA structures form spontaneously by single DNA strands through A–T and/or C–G Watson–Crick base pairing,^24,25 which provides a broad prospect for constructing complex nanoscale assemblies.

The unique structure, outstanding biocompatibility and robust stability of DNA-based SANs make them a competent tool for studying living cells. For instance, DNA origami named by Rothemund in 2006²⁶ exhibits various geometries such as cube,²⁷ tetrahedron,^28,29 octahedron,³⁰ bipyramid,³¹ dodecahedron,³² hollow sphere³³ and icosahedron.³⁴ These DNA hollow polyhedra with stable and intriguing structures have particular functions, thus broadening their applications in drug delivery, diagnostic biosensing, and smart therapeutics.^35,36

Encodable DNA structures allow researchers to control the formation of structures. For instance, DNA origami with controllable structures and functions could be manipulated as a nanomechanical imaging probe to detect single nucleotide polymorphism at the single-molecule level (Fig. 2A),³⁷ providing a better detection tool in the biosensing field. Qing and coworkers proposed a new strategy for sensitive mRNA detection in living cells based on the DNA tetrahedron, which is achieved by amplifying the signals through catalytic hairpin assembly,³⁸

DNA origami also can serve as a nanovehicle to carry drugs for targeted therapy. One project utilized DNA origami to engineer an autonomous DNA robot that is programmed to specifically transport payloads to tumor sites. After intravenous injection of the DNA nanorobots, thrombin molecules are specifically delivered to tumor-associated blood vessels, which induces intravascular thrombosis and ultimately leads to tumor necrosis. The immunological inertness and high therapeutic efficacy make DNA nanorobots promising candidates for precise cancer therapy.³⁹ Then, an addressable double-bundle DNA tetrahedron with specific modification sites is reported to co-deliver multiple functional components (nucleic acids and proteins) for efficient gene regulation (Fig. 2B).⁴⁰ Such self-assembled DNA nanostructures exhibit superior performance compared with that of other probes for the mechanically rigid and structurally stable formations, opening up new avenues for the research of intelligent drug delivery systems and bioimaging. However, the efficiency of gene therapy alone is not ideal, and it is possible to develop multiple therapeutic modalities synergistically based on this material at a later stage.

2.2. Self-assembled peptides

Peptides and peptide conjugates consist of dozens of amino acids, which play a vital role in biological systems, including activating related enzymes and regulating the physiological functions of cells. The peptides featuring precise selectivity, functionality, and biocompatibility can serve as perfect building blocks for the construction of diverse SANs such as nanotubes,⁴¹ nanofibrils,⁴² nanobelts⁴³ and hydrogels.⁴⁴ The chemical techniques for the synthesis of peptide self-assembly include solid-phase peptide synthesis, solution-phase synthesis, and ring-opening polymerization.⁴⁵ In order to impart specific functions to the peptide-assembled nanostructures, chemical modifications are required at particular sites. In recent years, peptide-based SANs have been extensively explored for biomedical imaging,⁴⁶ drug delivery,¹⁴ and biomedicine applications.⁴⁷

Manipulation of the structure and size of peptide-assembled SANs can protect them against protease degradation to reduce premature drug leakage. Lipid-like peptides consisting of an aspartic acid or hydrophilic lysine head and a hydrophobic tail resemble biological lipids in length and can form nanovesicles under physiological conditions.⁴⁸ Such peptide-assembled nanovesicles may be complementary to liposomes for drug delivery.

The assembly of peptides can redshift the fluorescence emission of peptides from the UV region to the visible region and endows the SANs with photostability, biocompatibility and narrow emission bandwidths, which is beneficial for bioimaging applications. Through further decoration of the targeting ligand, SANs can be selectively taken up by cancer cells to immediately release therapeutic drugs.⁴⁹ Crystalline dipeptide nanobelts, which are assembled via ultrasonic irradiation-driven solid–solid transformation, can act as a probe for polarization imaging of cells to visualize the uptake and fate of SANs, providing a noninvasive and cost-effective approach for visualizing live organisms (Fig. 3A).⁴³

Peptide-based SANs are widely utilized in biomedical applications especially in cancer therapy. Near-infrared (NIR)-absorbing photothermal SANs composed of biliverdin, short peptides, and metal ions are proposed. Such multifunctional photothermal nanomaterials may become a new generation of photothermal agents for clinical translation (Fig. 3B).⁵⁰ Another SAN composed of a CA inhibitor and a short peptide can target CA IX that is exclusively overexpressed on the membrane of hypoxic tumor cells, which helps to accelerate the assembly into nanofibers and facilitate cellular uptake of the assembly. During this process, the nanofiber may assemble into larger nanofiber bundles that pierce intracellular acid organelles and discriminatorily exert a significant inhibitory effect on hypoxic tumor cells (Fig. 3C).⁵¹ Peptide self-assembly has shown superior performance in bioimaging and biomedicine due to the following reasons: the self-assembly only involves one-step incubation and is therefore simple to prepare; the arrangement is mechanically rigid and structurally stable; the positive surface charge of these assemblies facilitates cellular uptake. Thus, peptide self-assembly with the above-mentioned advantages can be a useful tool for improving biosensing and biomedicine. However, the exorbitant price of peptides hinders their further development and is a major problem to be overcome for a multitude of future clinical applications.

2.3. Self-assembled polymers

Decades ago, researchers uncovered that amphiphilic molecules possess self-assembly behaviour. They form various morphologies that include sphere, cylinder, lamellae, and bicontinuous structures in aqueous solutions. There are also aggregates of polymers similar in size or resolution to small molecules.^52,53 Although the self-assembly of polymers and small molecules is based on similar principles, it is still necessary to construct polymer aggregates with precise and controllable structures for on-demand drug release.^9,54

Functional block copolymer-based SANs prepared by the polymerization-induced self-assembly strategy can also act as excellent nanocarriers with a prolonged circulation time, promoted passive accumulation and reduced systemic toxicity.^10,55,56 Therapeutic drugs can be separated from the biological microenvironment after encapsulation into the interior of nanoparticles but can be stimulated to release at the target.⁵⁷ Nanocarriers with benzaldehyde-functionalized cores are formulated via alcoholic reversible addition–fragmentation chain transfer (RAFT) dispersion polymerization, followed by doxorubicin (DOX) conjugation to the inner benzaldehyde through the condensation reaction to improve the loading efficiency. Benefiting from the acid-cleavable aromatic imine linkage, DOX can be readily released upon exposure to the acidic organelles, thereby exerting anticancer effects (Fig. 4A).⁵⁸ The vesicle can be endowed with some superior advantages through subtle design, including morphological stability and multiple selectivities. Pan fabricated a smart vesicle at up to 30% solid content via polymerization-driven self-assembly and reorganization. This intelligent vesicle exhibits pH-regulated permeability and the size exclusion effect that depends on the charge/size of the substance and the degree of membrane crosslinking induced by polymerization-induced self-assembly and reorganization (PISR) and UV irradiation (Fig. 4B), providing competitive nanocarriers for cargo payload.⁵⁹ A terrylenediimide–polyacrylic acid-based theranostic agent self-assembles into a macromolecular nanosystem with the advantages of high photothermal conversion efficiency, good biocompatibility, and robust structure that may practically solve the problems of the complicated nanostructure and instability facing nanomedicines.⁶⁰ Similarly, a hyperbranched polyporphyrin nanovesicle prepared by supramolecular polymerization-enhanced self-assembly is reported to enhance the efficiency of photothermal therapy (PTT) due to the regular stacking of polyporphyrin units inside the nanovesicle membranes.⁶¹

Self-assembled polymeric nanomaterials can also work as containers to carry diagnostic probes, fluorophores, or nanoparticles to build biosensing or bioimaging platforms. A straight-forward biointerface is fabricated by self-assembly of block copolymers into a core–shell structure which is further decorated with mannose. Attributed to the advantages of low cost, significant sensitivity and specificity, this nanostructure demonstrates as an innovative sensor for bacteria detection (Fig. 4C).⁶² Molecular imaging of abnormal metabolites is of particular importance for revealing the progression of disease and guiding intervention. For instance, the accumulation of hypochlorous acid can damage hepatoma cells and even cause cancers. In this context, a nanoprobe assembled from a single dye-based polymer is designed to precisely image hypochlorous acid in living cells (Fig. 4D).⁶³ The ultrafast response, excellent long-term stability, and prominent biocompatibility make this nanoprobe a promising platform for molecular imaging.

2.4. Self-assembled fluorescent organic SANs

Fluorescent organic SANs have shown outstanding potential in biosensing and bioimaging due to their advantages in terms of processability, low cost, and biocompatibility (Fig. 5A).⁶⁴ Their photophysical, chemical, and biological properties can be regulated by manipulating the arrangements of dyes. It can overcome the significant shortcomings of off-target effects, biological auto-fluorescence and low target-to-background ratios.

Recently, the emergence of luminescent molecules based on aggregation-induced emission (AIE)^65,66 and aggregation-induced quenching (ACQ)⁶⁷ techniques has motivated the application of AIE-/ACQ-based functional probes in bioimaging and cancer treatment. Organic fluorophores can form micelles for bioimaging and monitoring of drug release. By taking advantage of two fluorophores with completely opposite AIE and ACQ characteristics, a pH-responsive polymeric probe is engineered to trace the delivery process, guide photodynamic therapy (PDT) and visualize therapeutic response.⁶⁸ Then, an NIR AIE active nanoprobe is rationally designed for in situ high-resolution imaging of Alzheimer's disease-related amyloid-β (Aβ) plaques. Notably, this “off–on” probe contains a lipophilic π-conjugated thiophene-bridge for NIR emission and enhanced blood–brain barrier permeability. In this system, AIE signals are switched on at the target owing to the high affinity of the probe towards Aβ plaques, hence offering a favourable strategy for ultrasensitive mapping of Aβ plaques (Fig. 5B).⁶⁹ To further improve the sensitivity of the AIE probe, researchers constructed a near-infrared afterglow AIE nanoprobe with low background noise and reduced biological auto-fluorescence via energy transfer from Shaaps’ dioxane to the AIE emitter because of its delayed luminescence after light irradiation. In sharp contrast to conventional fluorescence imaging, it shows nearly 100-fold enhancement in the tumor-to-liver signal ratio, holding promise for precise imaging-guided cancer surgery.⁷⁰

High stability and favourable biocompatibility of nanomaterials are often prerequisites for successful nanomedicine. In this regard, polyethylene glycol (PEG) chains are incorporated into phototheranostic quaterrylenediimide (QDI) SANs for enhanced physiological stability and improved biocompatibility. Upon exposure to an 808 nm laser, the QDI SANs show strong NIR absorption but emit weak fluorescence and yield a low amount of reactive oxygen, thus contributing to a strong photothermal effect with a photothermal conversion efficiency of ∼64.7%.⁷¹ Based on the above efficient infrared imaging and therapeutic efficiency, it provides a tool reference for a more in-depth study of other materials with this light absorption property.

2.5. Self-assembled proteins

Proteins are one of the ubiquitous and essential biomolecules in biology and they widely exist in organisms ranging from unicellular prokaryotes to unicellular/multicellular eukaryotes. They are synthesized by specifically folding amino acid-composed polypeptide chains into three-dimensional structures. 20 kinds of amino acid components endow the proteins with versatile chemical functionalities.⁷² Particularly, protein-based SANs can improve the stability and mechanical strengths of protein, playing a vital role in regulating biological functions.

Proteins as natural drug carriers⁷³ can package anticancer drugs in the self-assembled form that includes drug-loaded self-assembled protein nanoparticles⁷⁴ and paclitaxel-/photosensitizer-/nanoparticle-/metal ion-induced self-assembly of proteins.^75–77 The self-assembly is often driven by the combination of π–π stacking, hydrophobic interaction, hydrogen bonding and ionic attraction.

Human serum albumin (HSA) nanoassemblies induced by a pH-responsive NIR dye are fabricated for real-time ratiometric photoacoustic pH imaging of tumors and pH-activated PTT.⁷⁸ Electrostatic assembly of proteins and charged polymers provides another effective strategy for protein assembly. Inspired by this, photosensitizer-encapsulated protein SANs with enhanced PDT efficacy are assembled using negatively charged HSA and positively charged poly-L-lysine followed by intermolecular disulphide cross-linking and surface PEGylation.⁷⁹ Besides, the capsid of cowpea chlorotic mottle virus (CCMV) can act as a nanocage to encapsulate photosensitizers through the interaction between negatively charged photosensitizers and positively charged CCMV protein,⁸⁰ providing a strategy for the construction of monodisperse and uniform organic photoactive nanospheres. Ferritin is another representative natural nanocarrier whose cavity can be utilized to embed enzymes.⁸¹ Such fusion protein not only retains the high catalytic activity of enzymes, but also enhances the tolerance of enzymes to proteolysis and heat. The above examples have led to a clearer understanding of the role of self-assembly in the construction and modulation of protein-based nanomedicines, providing new insights for the nanotechnology and pharmacological fields.

3. Self-assembled nanomaterials with different nanoparticles

Due to the unexpected optical features (e.g., chirality, fluorescence, surface-enhanced Raman resonance (SERS)), ease of entry into cells and intracellular stability, nanoassemblies consisting of multiple nanoparticle units (e.g., gold nanoparticles (Au NPs), gold nanorods (Au NRs), upconversion nanoparticles (UCNPs), silver nanoparticles (Ag NPs)) have many valuable applications.

3.1. Gold nanoparticles

Au NPs show synchronous absorbance and scattering when interacting with light. Plasmon electromagnetic fields are generated around the surface of the nanostructured metal. The changes in shape, size, composition, inter-particle interactions and dielectric constant of the surrounding medium can easily affect the characteristics of surface plasmon resonance (SPR)⁸² and create the property of chiral optical activity. Thus, Au NPs with tunable properties show increasing robustness in the field of biosensing. In addition, NIR absorbing Au NPs exhibit light-triggered drug or gene release behaviour, which is beneficial for the treatment and prevention of disease.^83,84

DNA represents a versatile and practical tool for precisely controlled assembly of nanoparticles.⁸⁵ DNA-mediated structural control can dynamically change the physicochemical properties of Au NPs.⁸⁶ Some works focus on the applications of DNA-driven Au NP assembly in biosensors.⁸⁷ One strategy is to fabricate DNA-driven gold heterodimers and apply their assembled circular dichroism to measure intracellular telomerase activity.⁸⁸ Low background interference and high signal-to-noise ratio are crucial for intracellular detection. In order to cope with the complex and dynamic physiological environment, a similar design combining Au NP dimerization and background-free Raman reporters is proposed for microRNA imaging in living cells (Fig. 6A).¹⁵ The DNA-mediated Au NP assembly is not only limited to the 2D arrangement, but also includes the 3D pattern.¹⁶ DNA is also widely explored as a molecular screw to gather and remodel colloidal nanomaterial systems. For example, DNA transforms the arrangement of the Au NP core surrounded by small satellites via the toe-hold displacement mechanism (Fig. 6B).⁸⁹ So, improving the efficiency of Au NP directed linkage has become crucial to promote its biological applications.

Besides, DNA-mediated Au NP assemblies have emerged as potent platforms for gene/drug delivery. Au NP as the core combines with FITC-labeled hairpin DNA sequences wherein drug molecules are intercalated. Under the stimulation of telomerase, the primers are extended, which leads to inner chain substitution and consequent fluorescence recovery/drug release, enabling the detection of telomerase activity and precise cancer treatment.⁹⁰

3.2. Gold nanorods

Au NRs have robust and tunable plasmon resonances that can be used for applications in biosensors and nanomedicine. Nanoscale plasmonic assemblies display extreme chiral optical activity. DNA molecules are commonly applied as chiral templates to connect nanoparticles to form chiral arrangements. For example, the side-by-side assemblies of Au NRs induced by the DNA target remarkably magnify the chiral bisignate plasmonic signals, thereby affording attomolar DNA biomarker detection.⁹¹ Some other reports use the chiral assembly of Au NRs for spatially resolved detection of intracellular miRNA.^92,93 The same goals can be accomplished by conjugating dye-labeled DNA hairpins onto the external surface of Au NRs.⁹⁴ The resulting luminescence recovery enables in situ imaging and quantitative detection of target mRNA.⁹⁵ By virtue of DNAzyme, the core–satellite superstructures can also be extended to the detection of metal ions.⁹⁶

With great advances in the fabrication and assembly of nanoparticles, a single discrete structure can realize multiple goals through self-assembly. By combining Au NRs with UCNPs, a dual-signal platform based on surface-enhanced Raman scattering and luminescence is developed for ultrasensitive and quantitative detection of miRNA and telomerase.⁹³ In addition, metal ion-responsive chiral satellite assemblies can be constructed by metal ion-specific DNAzyme, spiny platinum-covered Au NRs and UCNPs, which provides a promising probe for simultaneous detection of multiple intracellular divalent metal ions (Fig. 7A).⁹⁷ However, the connection efficiency of these mechanisms needs to be further strengthened.

The unexpected optical characteristics of Au NRs in terms of chirality and the Raman spectrum broaden their application in bioimaging and therapy. Based on this, a core–satellite therapeutic platform combining Au NRs and photosensitizer-anchored UCNPs performs as a powerful photothermal reagent and photosensitizer for synergistic photothermal/photodynamic therapy (Fig. 7B).⁹⁸ Considering that the precise diagnosis is crucial for tumor treatment, SANs made of platinum-coated Au NRs and Ag₂S are designed. The probes work as powerful diagnostic reagents for X-ray computed tomography (CT) imaging, PA imaging and miR-21-activated NIR-II fluorescence imaging to accurately locate tumors and guide PTT.⁹⁹ Furthermore, a signal amplification strategy is introduced into a similar nanosystem for more sensitive in vivo miRNA imaging and imaging-guided therapy.¹⁰⁰ Both of these offer promising therapeutic strategies for accurate diagnosis and tumor therapy.

3.3. Upconversion nanoparticles

Lanthanide-doped UCNPs have some outstanding merits such as a large anti-Stokes shift, low background fluorescence, minimal photodamage, ignorable biological toxicity, and high photostability. Particularly, the photostability of UNCPs makes them superior to commonly used fluorophores which often suffer from photobleaching, thus offering reliable signals for real-time monitoring. DNA can be precisely tethered and oriented on the surface of UCNPs.¹⁰¹ Benefiting from the optical features of UCNPs and DNA assembly technology, SANs assembled from UCNPs and DNA can be applied for bioimaging and cancer treatment (Fig. 8A).¹⁰²

As for bioapplications, DNA-driven assembled Au NP−UCNP pyramids can detect intracellular miRNA through dual-mode signals namely luminescence and plasmonic circular dichroism (CD) signals.¹⁰¹ This report demonstrates that the CD signal is much more sensitive to miRNA concentration than fluorescence signal. The self-assemblies formed by Au NPs and UCNPs are different from the principle of CD signal generated by the above self-assemblies formed by Au NRs and UCNPs; the former is generated by the circular dichroism between the double Au NRs, while the latter is generated by the overall heterogeneous structure. Similarly, Au–Au-UCNP ternary structures with luminescence/SERS activity are fabricated for simultaneous attomolar analysis of two disease markers.^103,104

UCNPs can convert NIR light into UV emission to cutoff the photocleavable bond, based on which activatable UCNP-DNA SANs are designed. Inspired by this, NIR-activated UCNPs@DNA nanodevices are rationally designed to detect intracellular ATP.¹⁰⁵ The design concept can also be applied for miRNA imaging with high space and time resolution, thereby expanding the toolbox of technologies for precise biological and medical analysis.¹⁰⁶ Furthermore, spatiotemporally controllable signal amplified mRNA imaging can be achieved by taking advantage of the hybridization chain reaction (HCR)-based signal amplification strategy (Fig. 8B).¹⁷ Given that metal ion-dependent DNAzymes are a class of artificial single-stranded DNA molecules with catalytic activities, the integration of DNAzyme and UCNPs enables the assembly to perform spatiotemporally controlled detection of metal ions in vivo.¹⁰⁷

Controllable stimuli-responsive drug release plays a vital role in cancer therapy. An UCNP-fueled DNA–azobenzene nanopump can rapidly convert NIR light to UV and visible emission to induce photoisomerization of azobenzene, which leads to unwinding and hybridization of DNA for on-demand drug release (Fig. 8C).¹⁰⁸ On this basis, this research group also constructed a NIR-activated miRNA amplifier consisting of UCNPs and photo-caged DNA nanocombs that undergoes a NIR photo-switched miRNA cascade reaction for precise PDT of early-stage tumors (Fig. 8D).¹⁸

3.4. Silver nanoparticles

The physical and chemical properties of Ag NPs such as surface chemistry, size distribution, shape characteristics, particle composition, capping/sealing layer, agglomeration profile, and dissolution rate are dedicated to various fields of application. Precise characterization of nanoparticles is essential because the physicochemical properties may have a vital impact on their biological activities¹⁰⁹ including bioavailability, biodistribution, cellular uptake and potential toxicity.¹¹⁰ So, it is necessary to expand the structures of Ag NPs in terms of size, morphology, and functionality to meet the requirements of diverse biomedical applications.¹¹¹

As for multiplexed detection of biomarkers, DNA frame-driven Ag NP self-assembled pyramids present some potential merits such as controllable discrete assembly-based analysis, switchable plasmonic coupling for SERS enhancement, reproducibility and ultrasensitivity. Particularly, stimuli-initiated conformation change is adopted to enhance the Raman signal for ultrasensitive quantification of multiple biomarkers (Fig. 9A).¹¹² When the chiral shell–satellites (Ag NPs@Au@Au NPs) are modified with chiral molecules, a small dihedral angle between the ellipsoidal shaped silver core and Au NPs is generated, contributing to excellent chiroplasmonic activities in the visible region. Under circularly polarized light illumination, the asymmetrical dipole–dipole structure outcompetes the core–satellite Ag NPs@Au NPs in the reactive oxygen species generation efficiency as well as the antitumor effect (Fig. 9B).¹¹³

Programmable SANs with remarkable CD signals are desirable for biological applications. A framework built with a shell core–satellite nanostructure and ochratoxin A aptamer presents a strong chiral signal manifested as an intense CD peak, enabling sensitive detection of mycotoxin ochratoxin A with a reasonable recovery rate.¹¹⁴ The synthesis and assembly process of Ag NPs should be carried out under strict control of the synthesis conditions in order to avoid a large toxicity of the probe itself in terms of biosensing, as it is also well used as a nanomedicine for antibacterial therapy.¹¹⁵ Biotoxicity of Ag NCs when used as an analytical tool is a key concern when assembling structures.

4. Conclusion and outlook

In this review, the recent advances of SANs in bioimaging, biosensing and cancer treatment are summarized. In order to further understand the functional mechanisms, we divide SANs into several parts according to their material compositions, including DNA-based assemblies, peptide-based assemblies, polymer-based assemblies, fluorescent organic-based assemblies, protein-based assemblies, and nanoparticle-based assemblies. The use of DNA links and controls the formation of diverse nanostructures, leading to the extension of various constructional concepts, mainly focusing on the ordered assembly of hybrid parts, nanostructures and biological functions.

Current investigations emphasize the design modernization, functions of the self-assembled platform, and fabrication of multi-unit heterogeneous nanostructures. Considering that most of the assembled systems still face shortcomings such as single imaging mode, limited penetration depth and low spatial resolution, multimodal detection with high spatiotemporal resolution is expected to provide vast space for accurate diagnosis and therapy. And the conformational transformation of SANs is expected to achieve real-time detection and multiple detection methods for biomarkers in deep tissues and organisms, as well as triggering the drug release to conduct controlled therapy. Furthermore, in order to cope with complex biological systems and sophisticated biological sample pretreatment, there is a need to further design and explore novel heterogeneous nanostructures for reliable, sensitive, and efficacious detection and treatment. Last but not least, the distribution and cytotoxicity of SANs have always been a major issue in diagnostic applications. Therefore, more fundamental investigation regarding biocompatible SANs with novel biosensing modalities and efficient therapeutic modalities should be performed to facilitate the well-being of humanity.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We gratefully acknowledge the financial support from the National Natural Science Foundation of China (21834004 and 21904063) and the China Postdoctoral Science Foundation (2018M640472 and 2019T120413).

Notes and references

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