Design And Bio-Imaging Applications Of Near-Infrared Fluorescent Polymer Nanoparticles

In vivo bio-imaging technology can provide critical physiological and pathological information and was widely used in biomedical research and clinical medicine.Through in vivo bio-imaging technology,various structural and functional information of patients can be non-invasively obtained,which is helpful for realizing the diagnosis and monitoring of diseases,even the guidance of surgery.In recent years,the development of fluorescence imaging technology has injected new vitality into the field of in vivo bio-imaging.Compared with the traditional imaging method,fluorescence imaging can capture images in a real-time and wide field of view,greatly improving the time resolution.Harmless optical radiation excitation and signal response greatly reduce damage to imaging targets.In addition,a wide variety of fluorescent probes enable a variety of functional imaging.The poor photon penetration depth in biological tissues is one of the key factors that restrict the development of in vivo fluorescence imaging.Related studies have shown that the scattering and absorption of photons by biological tissues,including the autofluorescence,is highly correlated with the wavelength of the photon.In the broad 700 nm–1700 nm NIR window,tissues exhibit extremely low scattering coefficients,absorption of photons and autofluorescence.Particularly,700 nm-900 nm is called Near-infrared I?NIR-I?window,and 1000 nm-1700 nm is called Near-infrared II?NIR-II?window.However,most organic fluorophores emit in the visible region,and current near-infrared fluorescent probes also suffer from low fluorescence quantum yield.Promising applications inspire the researchers to pursue new NIR fluorophores.Fluorescent conjugated polymer nanoparticles have attracted considerable attention in fluorescence imaging because of their tunable optical properties and superior biocompatibility.A significant advantage of polymer nanoparticles is their tunable optical properties.By designing the structure of the fluorescent conjugated polymer,adjustment of the excitation wavelength,the fluorescence wavelength,and the fluorescence quantum yield can be achieved.In this paper,we synthesized various fluorescent conjugated polymers which are suitable for in vivo biological imaging.The main research results are summarized as follows:1.Three fluorescent conjugated polymers with lower energy gaps were successfully synthesized by employ strong donor and acceptor structure.The polymers possessed a strong light absorption capacity(molecular absorption coefficient at 560 nm is 3×107 L mol^-1 cm^-1),deep red excitation and NIR fluorescence.Fluorescent dye NIR775 was selected to dope with polymers according to the spectral overlap.The optical properties of the nanoparticles are greatly improved after doping:firstly,the strong absorption capacity and deep red excitation?676 nm?are retained;secondly,the FWHM peak is narrowed from¹40 nm to²0 nm with an emission peak of 780 nm;and last,the fluorescence quantum yield is significantly improved from 2%to 21%.After labeled by the nanoparticles,MCF-7 cells show strong fluorescent signals in flow cytometry.Labeled MCF-7 cells were injected into the mice to simulate the cancer cells metastasis in blood.And the enrichment of cancer cells in the lungs was clearly detected by in vivo fluorescence imaging,which achieves long-term cell tracking in deep organs.2.In the nanoparticles,the polymer is normally in a state of aggregation.Due to the aggregation-caused quenching?ACQ?effects,the fluorescence quantum yield of the polymer would significantly reduce after preparing into nanoparticles.Accordingly,we employed the aggregation-induced emission?AIE?fluorophore phenothiazine to endow the AIE character to the polymer.The fluorescence quantum yield of polymer in tetrahydrofuran is around 9%,but substantially increase to 23%after preparing into nanoparticles.After coating with PS-PEG,the nanoparticles achieved targeted fluorescence imaging of the tumor by EPR effect,the tumor area demonstrated a high fluorescence signal with the signal-to-noise ratio of 3.2.3.Furthermore,we designed and synthesized 8 kinds of near-infrared AIE polymers.The fluorescence of these polymers covers from 600 nm to 1400 nm.In addition,the inter-and intra-chain interactions were successfully weakened by the introduction of steric hindrance side groups.As a result,nanoparticles fluorescence intensity was further enhanced based on AIE.Among these polymers,the fluorescence quantum yield of the NIR-II polymer?fluorescence peak at around 1100 nm?increased from 0.08%to 1.5%under such double fluorescence enhancement.This large increase demonstrates the effectiveness of such dual enhancement strategy.In addition,using NIR-II fluorescence polymer nanoparticle,the cerebrovascular structures were clearly visualized with an outstanding clarity to 0.11 mm by FWHM.Such excellent performance revels the potential of fluorescence dual-enhancement with AIE and anti-ACQ effect,which is a promising approach to achieve more superior NIR-II fluorophores.