Brightness of fluorescent organic nanomaterials

Brightness is a fundamental property of fluorescent nanomaterials reflecting their capacity to absorb and emit light. In sensing materials, brightness is crucial for high-sensitivity (bio)molecular detection, while in optical bioimaging it ensures high spatial and temporal resolution. Fluorescent organic nanoparticles (NPs) are particularly attractive because of their superior brightness compared to organic dyes. With the ever-growing diversity of organic nanomaterials, it is important to establish universal principles for measuring and estimating their brightness. This tutorial review provides definitions of brightness and describes the major approaches to its analysis based on ensemble and single-particle techniques. We present the current chemical approaches to fight Aggregation-Caused Quenching (ACQ) of fluorophores, which is a major challenge in the design of bright organic nanomaterials. The main classes of fluorescent organic NPs are described, including conjugated polymer NPs, aggregation-induced emission NPs, and NPs based on neutral and ionic dyes. Their brightness and other properties are systematically compared. Some brightest examples of bulk solid-state emissive organic materials are also mentioned. Finally, we analyse the importance of brightness and other particle properties in biological applications, such as bioimaging and biosensing. This tutorial will provide guidelines for chemists on the design of fluorescent organic NPs with improved performance and help them to estimate and compare the brightness of new nanomaterials with literature reports. Moreover, it will help biologists to select appropriate materials for sensing and imaging applications.

In Vivo Imaging Evaluation of Fluorescence Intensity at Tail Emission of Near-Infrared-I (NIR-I) Fluorophores in a Porcine Model

Over the last decade fluorescence-guided surgery has been primarily focused on the NIR-I window. However, the NIR-I window has constraints, such as limited penetration and scattering. Consequently, exploring the performance of NIR-I dyes at longer wavelengths (i.e., the NIR-II window) is crucial to expanding its application. Two fluorophores were used in three pigs to identify the mean fluorescence intensity (MFI) using two commercially available NIR-I and NIR-II cameras. The near-infrared coating of equipment (NICE) was used to identify endoluminal surgical catheters and indocyanine green (ICG) for common bile duct (CBD) characterization. The NIR-II window evaluation showed an MFI of 0.4 arbitrary units (a.u.) ± 0.106 a.u. in small bowel NICE-coated catheters and an MFI of 0.09 a.u. ± 0.039 a.u. in gastric ones. In CBD characterization, the ICG MFI was 0.12 a.u. ± 0.027 a.u., 0.18 a.u. ± 0.100 a.u., and 0.22 a.u. ± 0.041 a.u. at 5, 35, and 65 min, respectively. This in vivo imaging evaluation of NIR-I dyes confirms its application in the NIR-II domain. To the best of our knowledge, this is the first study assessing the MIF of NICE in the NIR-II window using a commercially available system. Further comparative trials are necessary to determine the superiority of NIR-II imaging systems.

Ultrabright Fluorescent Polymeric Nanofibers and Coatings Based on Ionic Dye Insulation with Bulky Counterions

Preparation of bright fluorescent materials based on polymers is hampered by a fundamental problem of aggregation-caused quenching (ACQ) of encapsulated dyes. Here, ultrabright fluorescent polymeric nanofibers and coatings are prepared based on a concept of ionic dye insulation with bulky hydrophobic counterions that overcomes the ACQ problem. It is found that bulky hydrophobic counterion perfluorinated tetraphenylborate can boost >100-fold the fluorescence quantum yields of cationic dye octadecyl rhodamine B at high loading (30 wt %) in biocompatible poly(methyl methacrylate) (PMMA). The concept is applicable to both rhodamine and cyanine dyes, which results in bright fluorescent polymeric materials of four different colors spanning from blue to near-infrared. It allows for preparation of electrospun polymeric nanofibers with >50-fold higher dye loading by mass (30 wt %, >20-fold higher molarity for rhodamine dyes) while preserving good fluorescence quantum yields (31%), which implies drastic improvement in their fluorescence brightness. The counterion-based polymeric materials are also validated as coatings of model medical devices, such as stainless steel fiducials and 3D-printed stents of complex geometry. Spin-coated fluorescent polymeric films loaded with a dye paired with bulky counterions exhibit excellent biocompatibility and low toxicity. Moreover, counterion-modified materials show much better stability against dye leakage in the presence of living cells and a serum-containing medium, compared to materials based on the dye with a small inorganic anion. Overall, by pushing the barriers of ACQ, our counterion approach emerges as a powerful tool to develop ultrabright fluorescent polymeric materials ranging from nano- and macroscale.

Deciphering protein microenvironment by using a cysteine specific switch-ON fluorescent probe

Fluorescent probes provide an unparalleled opportunity to visualize and quantify dynamic events. Here, we employ a medium-size, cysteine specific coumarin based switch-ON fluorescent probe 'L' to track protein unfolding profiles and accessibility of cysteine residues in proteins. It was established that 'L' is highly selective and exhibits no artifact due to interaction with other bystander species. 'L' is able to gauge subtle changes in protein microenvironment and proved to be effective in delineating early unfolding events that are difficult to otherwise discern by classic techniques such as circular dichroism. By solving the X-ray structure of TadA and probing the temperature dependent fluorescence-ON response with native TadA and its cysteine mutants, it was revealed that unfolding occurs in a stage-wise manner and the regions that are functionally important form compact sub-domains and unfold at later stages. Our results assert that probe 'L' serves as an efficient tool to monitor subtle changes in protein structure and can be employed as a generic dye to study processes such as protein unfolding.

Preoperative endoscopic marking of the gastrointestinal tract using fluorescence imaging: submucosal indocyanine green tattooing versus a novel fluorescent over-the-scope clip in a survival experimental study

BACKGROUND

Intraoperative localization of endoluminal lesions is can be difficult during laparoscopy. Preoperative endoscopic marking is therefore necessary. Current methods include submucosal tattooing using visible dyes, which in case of transmural injection can impair surgical dissection. Tattooing using indocyanine green (ICG) coupled to intraoperative near-infrared (NIR) laparoscopy has been described. ICG is only visible under NIR-light, therefore, it doesn't impair the surgical workflow under white light even if there is spillage. However, ICG tattoos have rapid diffusion and short longevity. We propose fluorescent over-the-scope clips (FOSC), using a novel biocompatible fluorescent paint, as durable lesion marking.

METHODS

In six pigs, gastric and colonic endoscopic tattoos using 0.05 mg/mL of ICG and markings using the fluorescent OSC were performed (T0). Simultaneously, NIR laparoscopy was executed. Follow-up laparoscopies were conducted at postoperative day (POD) 4-6 (T1) and POD 11-12 (T2). During laparoscopy, fluorescence intensity was assessed. In one human cadaver, FOSC was used to mark a site on the stomach and on the sigmoid colon, respectively. Intraoperative detection during NIR laparoscopy was assessed.

RESULTS

Gastric and colonic ICG tattooing and OSC markings were easily visible using NIR laparoscopy on T0. All FOSC were visible at T1 and T2 in both stomach and colon, whereas the ICG tattooing at T1 was only visible in the stomach of 2 animals and in the colon of 3 animals. At T2, tattoos were not visible in any animal. FOSC were still visible in both stomach and colon of the human cadaver at 10 days.

CONCLUSION

Endoscopic marking using FOSC can be an efficient and durable alternative to standard methods.

Solvatochromic Near-Infrared Probe for Polarity Mapping of Biomembranes and Lipid Droplets in Cells under Stress

Can polarity-sensitive fluorescent dyes monitor the response of live cells to fundamental stress conditions, such as deprivation from nutrition and oxidative stress? To address this question, we developed a push-pull dioxaborine probe (DXB-NIR) for biomembranes and lipid droplets featuring strong solvatochromism in the far-red to near-infrared region, high fluorescence brightness, photostability, and two-photon absorption cross section, reaching 13800 GM at 930 nm. In model membranes, DXB-NIR exhibits unprecedented 80 nm shift between liquid ordered and disordered membrane phases, allowing robust imaging of separated membrane microdomains. Two-color imaging of live cells with DXB-NIR enables polarity mapping in plasma membranes, endoplasmic reticulum, and lipid droplets, which reveals that starvation and oxidative stress produce an increase in the local polarity, and this change is different for each of the studied cell compartments. Thus, by pushing the limits of existing solvatochromic dyes, we introduce a concept of polarity mapping for monitoring the response of cells to stress.

A fluorogenic BODIPY molecular rotor as an apoptosis marker

Based on a BODIPY molecular rotor, we designed a probe that lights up its green fluorescence in apoptotic cells and distinguishes between early and late apoptosis.