Glycobiology is a rapidly evolving field that studies the structure, function, and biology of glycans, which basically are carbohydrate or sugar moieties that are found widely in nature. These moieties are present either as free-standing entities inside or on the cells or are covalently attached to other macromolecules as proteins or lipids (glycoprotein or glycolipids respectively). Together they form the human glycome that primarily is made up of 9 glycan building blocks arranged in distinct combinations alone or conjugated with other macromolecules, each having its unique stereochemical configuration in different spatial position in the cells. The human glycome is a complex system, as it exceeds the number of genes that are present in the human genome coding region by far. The only possible way to achieve this diversity in both kind and numbers is by not following the central dogma template for synthesis. Instead, the glycans are synthesized as secondary gene products by sequentially acting glycosidases and glycosyltransferases [1].

The glycans span across approximately 50% of all proteins in an organism and almost all the surface proteins in a cell (including receptor and adhesion proteins) [2]. The biological functions of glycans range from being non-significant to indispensable for the cells and organisms. They act as a communication system within the organism and between different organisms and their functions broadly fall into 4 categories (i) forming physical structures with unique modulatory properties (ii) extrinsic (interspecies) recognition by mediating interactions between different organism (iii) intrinsic (intraspecies) recognition by mediating cell-cell, cell-matrix, cell-molecule interactions (iv) molecular mimicry of host [2-5]. This versatility stresses the enormous potential of glycans for glycan based-drug discovery, therapeutics as well as biomarkers development [4,6,7]. Small molecular glycan therapeutics like heparin, Tamiflu, and Voglibose, to name a few, have existed since long [4]. However, the field of glycomedicine is evolving as new methods are developed to understand glycans and to harness its potential. In recent years the focus has shifted to solving biological and medical problems using glyconanotechnology, which synergistically combines the two-emerging field of glycobiology and nanotechnology [8]. Nanotechnology provides a platform of nanoscale materials having distinctive capabilities including high surface area to amplify the carbohydrate interactions, by making use of their unique optical, electronic, photonic, or magnetic properties [6,9,10]. The glycans provide biological versatility, activity, and recognition. Together they form the glycan nanoparticle (GNP) with the nanoparticle at the core and the glycan structures (mono-, oligo- or polysaccharides) as the shell [9]. Over the years, glyconanoparticles have revolutionized the biomedical research by developing platforms for cancer therapy, drug delivery, immunotherapy, biosensing and bioimaging [9,11]. Here we comment on the most used glycans, and nanomaterials for glyconanotechnology as previously discussed in our review [12] along with new insights.

Although the human glycome consists of numerous glycans, it essentially comprises of nine basic carbohydrates as follows: Glucose (Glc), Galactose, (Gal) Mannose, N-Acetylglucosamine (GlcNAc), N-Acetygalatosamine (GalNAc), L-Fucose (Fuc), Sialic Acid (Sia, Neu5Ac), Xylose (Xyl) and Glucuronic Acid (GlcA). These basic units can link to each other, to other proteins, and to lipids and be further oxidized, acetylated, phosphorylated, or sulfated increasing the diversity of the glycome. Glycans conjugated to lipids and protein, glycolipids and glycoproteins respectively, tend to be anchored to the cell membrane. Whereas proteoglycans which are composed of long linear chains of GAGs to protein backbones form the extracellular matrix, essential for intracellular signaling [13,14]. Glycans and glycosylation is such an essential process that aberrant or irregular glycosylation can be indicative of dysregulation in the cells. Multiple diseases and disorders including congenital disorders, cancer, autoimmune disorders, infectious and metabolic disease show irregular signature in the glycosylation pathways [15]. For example, in cancer, altered glycosylation can form neoantigens aiding in cancer metastasis and immune evasion [14]. These altered glycans and glycosylation patterns help target as well as differentiate normal vs cancer cells making use of glyconanotechnology specific and sensitive for drug delivery, imaging, diagnostics and for biomarker detection in cancer.

Along with the glycans, the group of specific proteins that bind to different glycans epitopes called glycan binding proteins (GBPs) also play a key role in maintaining signaling and homeostasis in the cells. They are majorly classified into Lectins and sulfated glycosaminoglycan (GAG)-binding proteins. Lectins bind terminal groups on glycans via specific carbohydrate recognition domain (CRDs) and are classified into families based on structural similarities. The sulfated GAGs GBPs bind to specific arrangements of carboxylic acid and sulfate groups along GAG chains and include heparan, chondroitin, dermatan and keratan sulfates [16]. When GBPs bind to their complementary glycans either on the cell surfaces or on the extracellular matrix they activate downstream signaling pathways through a series of steps. For example in immune cells, GBPs like siglecs, galectins, selectins, CD43 and CD45, binds to specific cell surface glycans and activate immune signaling pathways essential for the immune system to detect changes [13]. The cell surface glycan receptors as on human airway epithelium are also used by viral glycoprotein, hemagglutinin, to entry into the host cells initiating viral infections. This can be prevented by use of certain lectins that bind to these surfaces and display antiviral activity. Similarly, GBPs can contribute to anti-bacterial activities too [14].

Nanomaterials have been used in multiple different applications over the years and have had a profound impact on the field of not just biomedicine but also in various other scientific fields. Various nanomaterials have been used as scaffolds for glycans to support either drug delivery, therapy, biomarker sensing or imaging across various diseases and disorders. Some of the common nanomaterials used, especially in glyconanotechnology are discussed below and in Table 1.

Metal nanomaterials

Also known as inorganic scaffolds, metals have a characteristic optical, magnetic, and electronic properties that can be exploited for imaging, monitoring, and drug delivery. Metallic surfaces facilitate attachment and display of multiple ligands acting as signal transmitters and signal amplifiers especially useful in detection platforms. This display of ligands also can regulate physiological events in a cell, which is especially useful when designing GNTs for therapeutics [9].

Gold is the most used metal along with iron oxide. Gold is used for electrochemical assays, PET scans of blood brain barriers (BBB), and anti-cancer vaccine development. Gold nanorods are especially considered useful as optical biosensors. Gold glycan dye nanoprobes are used for detection of biomarkers [12]. Other metals such as silver and graphene oxide show better scattering efficiency and are also used for optical biosensor development [17]. Silver and zinc oxides glycan-nanoparticles are used to reduce cellular burden of viruses like SARS-CoV2 and H1N1 influenza. Other metals as selenium NP and palladium NP can inhibit bacterial growth and show anticancer activity respectively. Cadmium and copper metals have also been used as a scaffold for GNPs [14].

Although, metal NP are widely used they still require modifications to reduce cytotoxicity and to improve the low rate of clearance from the site of action [12].

Quantum dots

Semi-conductor quantum dots like other materials described in this review have active surface, unique photophysical properties and photostability [10]. They are useful for photodynamic therapy, labelling cells for imaging and detection of various analytes and viruses, when conjugated to metal and to glycans. It is also used for drug delivery and therapy as described in Table 1. Its inherent fluorescent properties make them versatile for Fluorescence Resonance Energy Transfer (FRET), Transmission Electron Microscopy (TEM), and other imaging techniques [18].

Carbon nanoparticles (CNPs)

Carbon nanotubes, graphene and carbon NPs are primarily useful for synthesis of biomolecules, imaging and for diagnosis as discussed in Table 1. They are relatively smaller in size and when combined with the glycans show improved water solubility and biocompatibility. They have distinctive electronic, mechanical, and thermal properties which make them versatile. Derivatives of CNPs as graphene, carbon nanotubes, carbon dots, and fullerenes support multivalency, flexibility, and spatial distribution of the glycans, making them ideal candidates for 2D and 3D scaffolds.

Carbon nanotubes and graphene nanoparticles are used for biosensing and drug delivery and when modified with Chitosan provide a surface for diagnostic modalities [19]. Graphene’s unique electrical conductivity and photoelectric properties are useful in monitoring systems and for early detection of cancer and other infectious diseases. Carbon dots, fullerenes and glycocarbon dots are a newer class of carbon-based fluorescent nanomaterial that are used as probes in cancer sensing applications and also have anti-bacterial applications. Fullerenes are another class of carbon NPs that have octahedral symmetry and globular structure, and are used as probes in enzyme inhibition, and in anti-viral, anti-bacterial and anti-cancer applications.

Newer and novel carbon nanoforms including but not limited to carbon nano onions, peapods, nanohorns, 2D graphene quantum dots are developed and used as scaffolds [19].

Dendrimer based glyconanoparticles which contain multivalent branched glycans attached to a dendritic polymer are used for cancer drug delivery. Metallo-glycodendrimer’s are with a metal core and are used in biomedical imaging and serve as a contrast agent. Polysaccharides like chitosan, hyaluronic acid and dextran are combined with a metal NP backbone to form polysaccharide based GNM with similar uses [3,19].

Different nanomaterials have distinctive characteristic which when combined with glycans creates a unique application. However, the number and combinations of glycans that can be used is also numerous making this whole technology versatile facilitating a new era of biomedical applications.