PLGA is one of the most extensively used biocompatible polymers for formulation of a nanoscale theranostic systems. The therapeutic moieties delivered by such formulations spans across drugs, proteins, RNA, DNA and peptides (Bouissou et al. 2006; Jain 2000; Ruhe et al. 2003). An interesting PLGA based theranostic system has been devised by Shi et al. wherein anticancer drug paclitaxel and MRI contrast magnetic nanoparticles were loaded inside PLGA carrier polymer. In order to further track the system under in vivo conditions fluorescent QDs were also grafted onto its surface. The iron oxide nanoparticle incorporated in the complex apart from providing contrast in MRI is also utilized for hyperthermia which synergizes the therapeutic effects of paclitaxel over the cancer cells. The theranostic system thus synthesized was conjugated with anti-prostate specific membrane antigen antibody for attaining targeted delivery to cancer cells alone. The system was validated under in vitro and in vivo systems for its cancer specific theranostic application (Cho et al. 2010).

The earliest reports of PLGA nanoparticles for theranostic application dates back to 2005 where McCarthy et al. used them for encapsulation and delivery of meso-tetraphenyl porpholactol which served the purposed of both imaging agent and cytotoxic agent when stimulated by external source. The complex was observed to be effective in curing cancers in mouse models with simple stimulation by external visible light source (McCarthy et al. 2005).

In another instance PLGA nanoparticles were utilized for overcoming multiple drug resistance with concomitant delivery of paclitaxel. In order to overcome drug resistance MDR1 gene silencer, P-gp targeted siRNA (viabiotin) was attached to PLGA which clearly exhibited an improved anti-cancer activity as compared to paclitaxel alone (Patil et al. 2010). Inclusion of a suitable imaging agent in such complexes can improve the diagnosis of drug resistant tumors which could be very much beneficial for efficient follow-up after cancer therapy. A similar system with targeted theranostic potential was deviced by Deepagan et al. where PLGA and PEG copolymer nanoparticles were used as carriers for delivery of camptothecin and ZnS QD. The complex was functionalized with antibody cetuximab for targeting it to the tumor site. The presence of therapeutic drug and imaging agent (i.e. theranostic) enables continuous follow-up of treatment for better management to treatment procedures (Deepagan et al. 2012). As an alternative to antibody, prostate-specific membrane antigen (PSMA) aptamers conjugated with biodegradable PEG-PLGA and PEG-poly(lactic acid) (PLA) micelles were synthesized by Farokhzad et al. for targeted delivery of docetaxel (Farokhzad et al. 2004, 2006). A specific drawback with PLGA is its imminant clearance from the circulatory system and in an attempt to overcome this often PEG moieties are grafted to PLGA which could have prolonged circulation time as it evaded the RES of the host (Akbarzadeh et al. 2012). In course with this, a PLGA-PEG based nanoparticles by the commercial name BIND-014 is being evaluated in clinical trials for controlled and targeted delivery of chemotherapeutic agents (Hrkach et al. 2012). Another group led by Liu reported synthesis of mPEG-PLGA-b-PLL block copolymer nanocapsules for delivery of anticancer drug adriamycin and siRNA. The complex antitumor efficacy was validated against Huh-7 hepatic carcinoma-bearing mice and was monitored simultaneously by NIR fluorescence dye Cy5 grafted onto its surface (Liu et al. 2012).

Chitosan is a natural biodegradable polymer with ionisable amino group favorable for functionalization with therapeutic and/or diagnostic molecules. A drawback of chitosan related polymer is its solubility which has led to synthesis of various other water soluble derivatives of chitosan like glycol chitosan and carboxy methyl chitosan. In one of the recent reports, Lee et al. have used glycol chitosan for the delivery of chlorin e6 (Ce6) which could serve the purpose of both in vivo imaging and photodynamic therapy. In order to further improve the Ce6 release profile, circulation half-life and its passive accumulation in tumors the Ce6 molecules were conjugated to hydrophobically modified glycol chitosan i.e. glycol chitosan-5β-cholanic acid. In vivo NIR imaging in mice models clearly indicated accumulation of the theranostic system at tumor sites. The subsequent prognosis of tumor after laser irradiation indicated instance of tumor necrosis and decline in tumor volume (Lee et al. 2011). The same carrier molecule (i.e. 5β-cholanic acid modified glycol chitosan) was utilized by Kyung et al. for encapsulation and delivery of hydrophobic anticancer drug camptothecin. The carrier could provide a sustained and prolonged release profile lasting for a week and also showed a tendency to passively accumulate at the tumor site when investigated in nude mice implanted with MDA-MB231 human breast cancer xenografts. A drastic reduction in tumor size was confirmed by near infrared fluorescence during the course of treatment (Kyung et al. 2008). In another independent work, glycol chitosan-cholanic acid nanoparticles were used to encapsulate and deliver doxorubicin and Bcl-2 siRNA in order to overcome drug resistance and facilitate an effective anticancer action of drug doxorubicin. The in vivo fate of both glycol chitosan based nanoparticles was monitored by non-invasive near infrared fluorescence imaging system (Fig. 2) (Yoon et al. 2014). The same amphipathic version of chitosan was shown to attain tumor targeted delivery by metabolic glycoengineering and click chemistry (Lee et al. 2014). The targeted delivery of precursor molecules by glycol chitosan-cholanic acid carrier molecule enabled tumor cells surface functionalization with azide groups. The following administration of copper-free click chemistry based anticancer drug efficiently localized their accumulation in the pre-functionalized (azide) tumor tissues.

Chitosan being laden with umpteen number of cationic amine groups is extensively used for delivery of nucleic acid to cancer cells. Inspite of its promising outcomes in delivery of siRNA and DNA to cancer cells under invitro conditions, chitosan in vivo application is limited due to interaction with hyaluronic acids present in the extracellular matrix. Thus, in order to overcome this, Ki et al. worked out on improving the physical stability of chitosan complexes. The authors proposed hybrid nanocomplexes of chitosan protamine, lecithin and thiamine pyrophosphate for delivery of surviving siRNA to Prostatic Small Cell Carcinoma (PC3). A marked decline (i.e. 21.9 %) in SVN expression was observed which suggested tumor targetability and growth inhibitory effects of the composite system (Ki et al. 2014). Another modified form of chitosan, thiolated chitosan nanoparticles were used by Anitha et al. for combined delivery of curcumin and 5-fluorouracil to colon cancer cells. The drug pharmacokinetics and biodistribution was evaluated further in Swiss Albino mouse models. The system demonstrated improved bioavailability of drugs in the blood plasma with simultaneous combinatorial anticancer effects of curcumin and 5-fluorouracil (Anitha et al. 2014).

A group headed by Deng et al. employed hyaluronic acid conjugated chitosan nanoparticles (HA-CS NPs) for delivering miR-34a (a tumor suppressive molecule in breast cancer) and doxorubicin to breast cancer cells with special emphasis on overcoming metastatic relapse. The study also showed the synergistic anticancer effects attained by codelivery of miR-34a and doxorubicin by HA-CS NPs (Deng et al. 2014).

In order to attain stimulated release of drug in a cancer specific milieu, a pH sensitive and temperature responsive poly(N-isopropylacrylamide)-chitosan nanohydrogels were synthesized by Jaiswal et al. Fe₃O₄ magnetic nanoparticles were also incorporated in nanohydrogel for hyperthermia for temperature stimulation of the composite system by AC magnetic field (AMF). The anticancer potentials of the therapeutic system was validated against human breast (MCF-7) and cervical carcinoma (HeLa) cells under in vitro conditions, which was 35–45 % in HeLa cells and 20–70 % in MCF-7 cells depending upon the AMF applied (Jaiswal et al. 2014).

A chitosan derivative, N-Carboxyethyl chitosan ester (CS-EA) was synthesized by another group to attain aptamer (MUC1 DNA) mediated targeted delivery of SN38 to colon cancer. The anticancer potentials of the system was investigated against MUC1 positive cell line and the corresponding cellular uptake and targeting was confirmed by confocal microscopy (Yoon et al. 2014). Poly(ethylene glycol) grafted chitosan copolymer was conjugated with tumor targeting siRNA/folic acid to form a theranostic nanoformulation. The theranostic system attained better compatibility with erythrocytes and effectively inhibited proliferation by gene knockout in BALB/c mice bearing OVK18 tumor xenograft by in vivo imaging. The preliminary in vitro studies were well complimented with flow cytometry RT-qPCR and western blot analysis for transfection and gene silencing activity (Li et al. 2014). Glycol chitosan has also been modified with different extents of hydrophobic N-acetyl histidine (NAcHis-GC) which is tagged with I¹³¹. Similarly I¹³¹ tagged doxorubicin was also encapsulated within NAcHis-GC for continuous monitoring using a gamma camera. The same system was evaluated under in vivo conditions with near-infrared fluorescence Cy5.5-labeled NAcHis-GC. The therapeutic outcomes of NAcHis-GC nanoparticles loaded with doxorubicin was compared with other alternative carriers reported thus far and was verified against xenograft mice models (Lee et al. 2010).

It is clear from afore mentioned examples that polymers have revolutionized the field of cancer therapy and diagnostics. The targeted delivery of two or more synergistic drugs to cancer cells with simultaneous real-time imaging system would be sought after as the future theranostic system for efficient management of cancer.

Dendrimers represent a class of chemically synthesized globular molecules with very well-defined structures. From the view of polymer chemistry, dendrimers are nearly defect-free, monodisperse (meaning consistent size and form) moieties with highly branched three-dimensional structure. Graphically, their architecture and dimensions resemble to small proteins and hence sometimes referred to as artificial proteins. The structural components of dendrimers include a central core from which branches (interior layers) emanate in an ordered fashion. The terminal groups attached to the outermost interior generations are mainly responsible for the functionality of the dendrimer (Lee et al. 2005) (Fig. 3). Dendrimers are produced by a series of repetitive chemical reactions, in which each additional iteration leads to a new layer i.e. ‘generation’ with double the number of active sites (peripheral end groups) and approximately twice the molecular weight of preceding generation. These moieties gained conception in the late 1970s and early 1980s with the pioneering works of Tomalia, Vogtle, Denkewalter, Newkome and co-workers. Tomalia and group demonstrated the iterative coupling of ethylene diamine to a central ammonia core to form various branched macromolecules and entitled them as ‘starburst dendrimers’ or poly(amidoamine) PAMAM dendrimers (Tomalia et al. 1985). Vogtle and co-workers, studied the controlled synthesis of dendritic units to produce polymeric macromolecules with large cavities and designated them as ‘cascade molecules’ (Vogtle et al. 1978). The first ever dendritic wedge with lysine residues at the branching points was reported by Denkewalter et al. (1981). Newkome’s group called dendrimers as “arborols” (Latin ‘arbor’ means a tree) (Newkome et al. 1985). Synthesis of dendrimers by specific chemical reactions is an appropriate example of controlled-hierarchical synthesis, an approach that enables ‘bottom–up’ creation of intricate systems. Initially, the researchers emphasized on understanding the synthesis, chemical and physical properties of dendrimers, but recently the focus has shifted to exploring the potential biological applications of dendrimers. Recently, dendrimers offer wide scope in various fields ranging from drug/gene delivery to cellular imaging to development of vaccines, antibacterial, antiviral and anticancer agents. (Aulenta et al. 2003; Stiriba et al. 2002; Patri et al. 2002; Boas and Heegaard 2004).

The unique molecular architecture of dendrimer macromolecules impart those significantly improved physicochemical properties when compared to linear polymers. In solution, linear chains exist as random coils; while dendrimers tend to stay as dense shells. Moreover, the exterior functional groups of dendrimers impart them with properties of high solubility, miscibility and reactivity. The structural versatility and controlled multivalence of dendrimers can be used to perform multiple conjuagtion reactions to attach several drug molecules, targeting moieties and solubilizing groups to the dendrimer periphery. Moreover, dendrimers with low polydispersity contrarily to some linear polymers with distinct molecular weight can provide reproducible pharmacokinetic behavior. Such interesting properties make dendrimers highly suitable for biological applications (Duncan et al. 2001).

Although various techniques are prevalent in clinical trials for cancer therapy such as surgery, chemotherapy and radiation therapy, etc. Nevertheless cancer therapeutics and diagnostic techniques also benefiting from components that efficiently absorb light (Photonics) includes photothermal and photodynamic therapy (Dolmans et al. 2003; Haung et al. 2006), optical frequency domain imaging (Vakoc et al. 2009), fluorescent and colorimetric detection (Chan and Nie 1998; Storhoff et al. 2004), photoacoustic tomography (also known as optoacoustic tomography) (Wang 2009; Xu et al. 2006), and multimodal techniques (Weissleder and Pittet 2008), etc. Optically active inorganic nanoparticles often interacts strongly with photons thus can be used as agents for above mentioned techniques.

Among the fluorescent probes, quantum dots are valuable and have extinction coefficients in the range of 105–106 M⁻¹ cm⁻¹ (Klostranec and Chan 2006). Gold nanoparticles (GNPs) are also useful for photothermal, colorimetric detection and photoacoustic techniques owing to their much higher extinction coefficients in the order of 109–1,011 M⁻¹ cm⁻¹ (Yguerabide and Yguerabide 1998). Despite being widely studied, they lack extensive medical execution, probably due to lack of concern about the long term safety and drug loading capacity typically being restricted to the nanoparticle surface (Ghosh et al. 2008; Nel et al. 2006). Contrarily, organic nanoparticles (including liposomes, nanospheres micelles, lipoproteins, and polymersomes) have shown several applications owing to their drug delivery potential and robust biocompatibility (Peer 2007). Though, intrinsically their use in biophotonics was limited due to lack of light absorption in the near-infrared region. Although various supramolecular assemblies has been formed entirely by porphyrin conjugates (intensely light-absorbing organic small molecules) though these constructs lacks stability, solubility or biological utility which constraint their use as biophotonic agent (Drain 2009). Hence nanotechnology introduces ‘porphysomes’; organic nanoparticles formed by self-assembled phospholipid-porphyrin conjugates which display liposome like structure and loading capacity, excellent biocompatibility, structure dependent fluorescence quenching, and having high absorption of near-infrared light (NIR), and demonstrate potential for various biophotonic applications (Lovell et al. 2011).

Carbon dots (C-dots) are the newly discovered fluorescent nanomaterials that constitute a new class of nanocarbon family with quasispherical shape and sizes below 10 nm. C-dots have occupied the centre stage for fluorescence related applications due to their size, cost and abundant nature. Carbon is generally a black material and has low solubility and fluorescence. On the other hand, C-dots have strong fluorescence and due to this reason they are sometimes referred to as ‘fluorescent carbon’. The existence of C-dots came to light when researchers were trying to purify single-walled carbon nanotubes (SWCNTs) through preparative electrophoresis in 2004. Accidently, they observed a fast-moving band of highly luminescent carbonaceous material which was certainly not the SWCNTs they were looking for (Xu et al. 2004). Typically, C-dots display some unique properties which include size and excitation dependent emission, multicolour emission, strong fluorescence, broad excitation spectra, high quantum yield and longer fluorescence lifetimes. These features have attracted the use of C-dots for a host of prospective applications, such as sensing, photocatalysis, optoelectronics and energy storage (Baker et al. 2010). In contrast to traditional metal based semiconductor quantum dots (QDs) and organic dyes, C-dots exhibit some remarkable features in terms of being environmentally and biologically benign, high aqueous solubility, colloidal stability, high photostability, ease of surface functionalization and chemical inertness. Consequently, C-dots are gaining grounds for biological applications and emerging as the most sought after alternative to QDs for similar fluorescence related applications. This area is developing fast, with a considerable number of breakthroughs taking place in the recent years. The first complete study regarding the origin and fluorescence of C-dots was provided by Sun et al. (2006). They employed oligomeric species for rendering effective passivation of the dot’s surface, resulting in bright fluorescence emissions (Fig. 17). It was thus found that surface passivation has implications for C-dots to have strong fluorescence.

However, there have been reports of non-passivated C-dots exhibiting fluorescence, but they generally have low quantum yields. The synthetic approaches for C-dots can be categorized into top–down and bottom–up approaches. Top–down approaches include arc discharge, electrochemical and laser ablation which involve the breakdown of chunks of carbon structure, which are quite complicated processes and require energy-consuming devices (Baker and Barker 2010). In comparison, the bottom–up approaches are inexpensive and involve the synthesis of C-dots from molecular precursors by means of chemical reactions like microwave (Sachdev et al. 2013), solvothermal (Mitra et al. 2013) and hydrothermal (Sachdev et al. 2014; Yang et al. 2014) treatments. In addition, C-dots may come from a variety of unconventional sources such as organic matter, natural products such as orange juice, soyabean etc. (Li et al. 2013; Liu et al. 2007; Sahu et al. 2012). This in turn suggests the simplicity and generality of preparative protocols for C-dots synthesis. There are abundant reports depicting one-step synthesis of C-dots through simple kitchen chemistry techniques. For instance, Palashuddin et al. (2012) demonstrated the presence of amorphous carbon nanoparticles of 4–30 nm size by heating daily food items such as bread, jaggery, cornflakes, or biscuits which exhibited photoluminescent emissions. In another study, the same group reported a facile method of synthesis of C-dots using a commercially available induction coil heater by using an aqueous solution of citric acid and a diamine compound (Palashuddin et al. 2014). Generally, the synthetic strategies for C-dots involve the use of a single precursor that acts as both carbon source and a passivating agent or separately employing a passivating agent in addition to carbon precursor. Wang et al. reported the synthesis of C-dots from glycerol with the aid of small amount of an inorganic ion (phosphate solution) without any surface passivation reagent (Wang et al. 2011). A rapid, one-step microwave mediated method for synthesizing C-dots using poly(ethylene glycol) (PEG) as a carbon source as well as a passivating agent has also been reported (Jaiswal et al. 2012). Yang et al. (2014) employed hydrothermal method to synthesize nitrogen-doped C-dots using ammonium citrate, serving as both carbon source as well as meeting the requirements of surface passivation. The groups on the surface of N-doped CDs acted as a self-passivation layer. Polyethylene glycol (PEG), polyethyleneimine (PEI), poly(ethylenimide)-co-poly(ethyleneglycol)-co-poly(ethyl-enimide) (PPEI), 4,7,10-trioxa-1,13-tridecanediamine (TTDDA) are some of the commonly employed agents for surface passivation. Nevertheless, out of the above, the attachment of nitrogen containing moieties onto the surface of C-dots has been found to generate stronger fluorescence emission. Peng et al. (2009) prepared C-dots by dehydration and oxidation of carbohydrates with sulphuric acid and nitric acid respectively, which were weakly emissive. Quantum yield increased to 13 % after passivation with TTDDA. Sachdev et al. (2013) demonstrated a novel one-step method for synthesizing C-dots using chitosan as a carbon source and PEG as a passivating agent through microwave mediated reaction, but the reported quantum yield was lower. In a study of similar relevance, the same group made a comparative analysis between PEG and PEI passivated C-dots synthesized by one-step hydrothermal method, without any post-synthetic treatment (Sachdev et al. 2014). Consequently, the quantum yield of C-dots synthesized by hydrothermal treatment was more compared to those synthesized by microwave treatment. These results suggest that the synthesis of brightly fluorescent C-dots is linked to the selection of right synthetic method along with the passivation polymer. Another interesting scheme of improving the fluorescence emission from C-dots involves the surrounding of C-dots by a metal-containing shell or its association with a metal-based nanostructure. For example, core carbon nanoparticle surface was doped with inorganic salts (ZnO, ZnS, or TiO₂) along with the organic functionalization (Baker and Baker 2010; Luo et al. 2013). The resulting C-dots (C_ZnO-dots, C_ZnS-dots, or -dots) exhibited much brighter fluorescence emissions than their undoped counterparts. All the above investigations point out that C-dots contain a carbon nanoparticle core and surface passivation is responsible for tuning the fluorescence performance. The most widely accepted emission mechanism in C-dots is the radiative recombination of surface-confined electrons and holes. Surface passivation schemes make surface sites more active and emissive to facilitate effective recombinations (Sachdev et al. 2013; Sun et al. 2006).