Selective delivery of imaging probes and therapeutics to the endoplasmic reticulum or Golgi apparatus: Current strategies and beyond

To maximize therapeutic effects and minimize unwanted effects, the interest in drug targeting to the endoplasmic reticulum (ER) or Golgi apparatus (GA) has been recently growing because two organelles are distributing hubs of cellular building/signaling components (e.g., proteins, lipids, Ca2+) to other organelles and the plasma membrane. Their structural or functional damages induce organelle stress (i.e., ER or GA stress), and their aggravation is strongly related to diseases (e.g., cancers, liver diseases, brain diseases). Many efforts have been developed to image (patho)physiological functions (e.g., oxidative stress, protein/lipid-related processing) and characteristics (e.g., pH, temperature, biothiols, reactive oxygen species) in the target organelles and to deliver drugs for organelle disruption using organelle-targeting moieties. Therefore, this review will overview the structure, (patho)physiological functions/characteristics, and related diseases of the organelles of interest. Future direction on ER or GA targeting will be discussed by understanding current strategies and investigations on targeting, imaging/sensing, and therapeutic systems.

Subcellular targeting strategies for protein and peptide delivery

Cytosolic delivery of proteins and peptides provides opportunities for effective disease treatment, as they can specifically modulate intracellular processes. However, most of protein-based therapeutics only have extracellular targets and are cell-membrane impermeable due to relatively large size and hydrophilicity. The use of organelle-targeting strategy offers great potential to overcome extracellular and cell membrane barriers, and enables localization of protein and peptide therapeutics in the organelles. Although progresses have been made in the recent years, organelle-targeted protein and peptide delivery is still challenging and under exploration. We reviewed recent advances in subcellular targeted delivery of proteins/peptides with a focus on targeting mechanisms and strategies, and highlight recent examples of active and passive organelle-specific protein and peptide delivery systems. This emerging platform could open a new avenue to develop more effective protein and peptide therapeutics.

A naphthalimide-based fluorescent platform for endoplasmic reticulum targeted imaging

A series of naphthalimide dyes (TRNATR, MOTNAMOT, MPNAMP, TYNATY, PNAP and IZNAIZ) were designed and synthesized by altering the side chains of the naphthalimide. Without the need for ER-targeting groups, the first five dyes were found to specifically target the ER, likely due to their well-suited lipophilic properties. Furthermore, TRNATR and TYNATY were proven effective for studying ER stress, showing promise in tracking ER autophagy in living cells triggered by tunicamycin and nutritional starvation.

Multicolor Tuning of Perylene Diimides Dyes for Targeted Organelle Imaging In Vivo

The adjustment of the emission wavelengths and cell permeability of the perylene diimides (PDI) for multicolor cell imaging is a great challenge. Herein, based on a bay-region substituent engineering strategy, multicolor perylene diimides (MCPDI) were rationally designed and synthesized by introducing azetidine substituents on the bay region of PDIs. With the fine-tuned electron-donating ability of the azetidine substituents, these MCPDI showed high brightness, orange, red, and near infrared (NIR) fluorescence along with Stokes shifts increasing from 35 to 110 nm. Interestingly, azetidine substituents distorted to the plane of the MCPDI dyes, and the twist angle of monosubstituted MCPDI was larger than that of disubstituted MCPDI, which might efficiently decrease their π-π stacking. Moreover, all of these MCPDI dyes were cell-permeable and selectively stained various organelles for multicolor imaging of multiple organelles in living cells. Two-color imaging of lipid droplets (LDs) and other organelles stained with MCPDI dyes was performed to reveal the interaction between the LDs and other organelles in living cells. Furthermore, a NIR-emitting MCPDI dye with a mitochondria-targeted characteristic was successfully applied for tumor-specific imaging. The facile synthesis, excellent stability, high brightness, tunable fluorescence emission, and Stokes shifts make these MCPDI promising fluorescent probes for biological applications.

Intricate subcellular journey of nanoparticles to the enigmatic domains of endoplasmic reticulum

It is evident that site-specific systemic drug delivery can reduce side effects, systemic toxicity, and minimal dosage requirements predominantly by delivering drugs to particular pathological sites, cells, and even subcellular structures. The endoplasmic reticulum (ER) and associated cell organelles play a vital role in several essential cellular functions and activities, such as the synthesis of lipids, steroids, membrane-associated proteins along with intracellular transport, signaling of Ca2+, and specific response to stress. Therefore, the dysfunction of ER is correlated with numerous diseases where cancer, neurodegenerative disorders, diabetes mellitus, hepatic disorder, etc., are very common. To achieve satisfactory therapeutic results in certain diseases, it is essential to engineer delivery systems that can effectively enter the cells and target ER. Nanoparticles are highly biocompatible, contain a variety of cargos or payloads, and can be modified in a pliable manner to achieve therapeutic effectiveness at the subcellular level when delivered to specific organelles. Passive targeting drug delivery vehicles, or active targeting drug delivery systems, reduce the nonselective accumulation of drugs while reducing side effects by modifying them with small molecular compounds, antibodies, polypeptides, or isolated bio-membranes. The targeting of ER and closely associated organelles in cells using nanoparticles, however, is still unsymmetrically understood. Therefore, here we summarized the pathophysiological prospect of ER stress, involvement of ER and mitochondrial response, disease related to ER dysfunctions, essential therapeutics, and nanoenabled modulation of their delivery to optimize therapy.

Regioselectivity Control in Spirobifluorene Nitration under Mild Conditions: Explaining the Crivello's Reagent Mechanism

The regioselective nitration of 9,9'-spirobifluorene under mild conditions is reported for the first time by operating under Menke's and Crivello's conditions. The optimized protocol allows obtaining 2-nitro and 2,2'-dinitro-9,9'-spirobifluorene in yields of 79 and 95% and, for the first time, 2,2',7-trinitro-9,9'-spirobifluorene with 66% yield. Besides, the role of dinitrate salt in Crivello's protocol has been now clarified, which opens novel scenarios in the preparation of functional materials.

A HBDI-Based Fluorescent Probe for Labeling Endoplasmic Reticulum in Living Cells

The endoplasmic reticulum (ER) is an important organelle in eukaryotic cells and is closely involved in the synthesis and processing of proteins, as well as the storage, regulation, and release of calcium. A series of signaling pathways within the ER play a crucial part in the pathogenesis of various diseases, including cancer. Thus, it is necessary to design ER-targeting probes to monitor these signaling pathways. Additionally, precision medicine also requires new ER-targeting group to facilitate the delivery of drug cargoes to the ER. However, only a limited number of ER-targeting groups have been used for the design of fluorescent probes for ER imaging in living cells, as well as the development of ER-targeted drug delivery systems. Herein, a new ER-targeting fluorescent probe (BDI-ER) was designed and prepared. BDI-ER contains the hydrophilic fluorophore, derived from the core structure of GFP, and two hydrophobic octadecane chains. The amphipathic nature of BDI-ER facilitates localization in the ER. Live cell imaging demonstrated selective localization of BDI-ER towards ER compared to other organelles. Additionally, co-localization imaging in various cell lines indicate that BDI-ER is effective at targeting the ER.

Fluorescent probes for targeting endoplasmic reticulum: design strategies and their applications

Advances in developing organic fluorescent probes and fluorescence imaging techniques have enhanced our understanding of cell biology. The endoplasmic reticulum (ER) is a dynamic structure that plays a crucial role in protein synthesis, post-translational modifications, and lipid metabolism. The malfunction of ER contributes to several physiological and pathological conditions. Therefore, the investigations on the imaging and role of ER have attracted a lot of attention. Due to their simplicity, synthetic tunability, photostability, high quantum yields, easier cellular uptake, and lower cytotoxicity, organic fluorophores offer invaluable tools for the precision targeting of various cellular organelles and probe ER dynamics. The precision staining is made possible by incorporating specific functional groups having preferential and local organelle biomolecular interactions. For instance, functional moieties such as methyl sulfonamide, sulfonylurea, and pentafluorophenyl assist in ER targeting and thus have become essential tools to probe a deeper understanding of their dynamics. Furthermore, dual-function fluorescent probes that simultaneously image ER and detect specific physiological parameters or biological analytes were achieved by introducing special recognition or chemically reactive sites. This article attempts to comprehensively capture various design strategies currently employed by researchers utilizing small organic molecules to target the ER and detect specific analytes.

Strategies for organelle targeting of fluorescent probes

Fluorescent tools have emerged as an important tool for studying the distinct chemical microenvironments of organelles, due to their high specificity and ability to be used in non-destructive, live cellular studies. These tools fall largely in two categories: exogenous fluorescent dyes, or endogenous labels such as genetically encoded fluorescent proteins. In both cases, the probe must be targeted to the organelle of interest. To date, many organelle-targeted fluorescent tools have been reported and used to uncover new information about processes that underpin health and disease. However, the majority of these tools only apply a handful of targeting groups, and less-studied organelles have few robust targeting strategies. While the development of new, robust strategies is difficult, it is essential to develop such strategies to allow for the development of new tools and broadening the effective study of organelles. This review aims to provide a comprehensive overview of the major targeting strategies for both endogenous and exogenous fluorescent cargo, outlining the specific challenges for targeting each organelle type and as well as new developments in the field.

Viscosity sensitive endoplasmic reticulum fluorescent probes based on oxazolopyridinium

A series of viscosity sensitive fluorescent probes 1a-e were synthesized by linking coumarin and oxazolopyridinium via dimethylene in this paper. The viscosity test of probes 1a-e indicated that the fluorescence intensity of the probes enhanced significantly with the increase of viscosity of the system (0.89-865 cP), and exhibited a nearly OFF-ON response to viscosity at 648 nm, 650 nm and 650 nm, respectively. In addition, cells still had a high survival rate after co-culturing with probes 1a-e for 12 h (94-98%). Meanwhile, the laser confocal experiment showed that the variation of the carbon chain length in the oxazolopyridinium could affect the subcellular region of the localization of the probes in cells. When the length of the carbon chain in oxazolopyridinium was between n-C7H15 and n-C12H23, probes 1b-d had the ability to target the endoplasmic reticulum in the cells. Moreover, probes 1b-d showed no significant change in fluorescence intensity after 35 min of continuous laser confocal irradiation, indicating that they had excellent anti-photobleaching properties.

Switchable cascade C-H annulation to polycyclic pyryliums and pyridiniums: discovering mitochondria-targeting fluorescent probes

Disclosed herein is a counterion additive-switched rhodium-catalyzed cascade triple C-H annulation of 4-hydroxy-1-naphthaldehydes with alkynes, in which six chemical bonds are formed in one-pot. This reaction enables the rapid assembly of diverse polycyclic pyrylium and pyridinium fluorophores, which leads to two specific mitochondria-labeling reagents with low cytotoxicity and superior photostability.

New β-carboline fluorophores with superior sensitivity and endoplasmic reticulum specificity for tracking ER changes

Autofluorescing carboline-fluors were efficiently and rationally developed via a Pictet-Spengler involved one-pot multi-component reaction. The carboline-fluors demonstrate specific targeting towards the endoplasmic reticulum in living cells, and superior sensitivity to commercial ER-Trackers. Importantly, they were also successfully used to visualize changes in the ER during cell apoptosis and ER stress.

Mechanosensitive Fluorescent Probes to Image Membrane Tension in Mitochondria, Endoplasmic Reticulum, and Lysosomes

Measuring forces inside cells is particularly challenging. With the development of quantitative microscopy, fluorophores which allow the measurement of forces became highly desirable. We have previously introduced a mechanosensitive flipper probe, which responds to the change of plasma membrane tension by changing its fluorescence lifetime and thus allows tension imaging by FLIM. Herein, we describe the design, synthesis, and evaluation of flipper probes that selectively label intracellular organelles, i.e., lysosomes, mitochondria, and the endoplasmic reticulum. The probes respond uniformly to osmotic shocks applied extracellularly, thus confirming sensitivity toward changes in membrane tension. At rest, different lifetimes found for different organelles relate to known differences in membrane organization rather than membrane tension and allow colabeling in the same cells. At the organelle scale, lifetime heterogeneity provides unprecedented insights on ER tubules and sheets, and nuclear membranes. Examples on endosomal trafficking or increase of tension at mitochondrial constriction sites outline the potential of intracellularly targeted fluorescent tension probes to address essential questions that were previously beyond reach.

BODIPY with Tuned Amphiphilicity as a Fluorogenic Plasma Membrane Probe

Staining of the plasma membrane (PM) is essential in bioimaging, as it delimits the cell surface and provides various information regarding the cell morphology and status. Herein, the lipophilicity of a green emitting BODIPY fluorophore was tuned by gradual functionalization with anchors composed of zwitterionic and aliphatic groups, thus yielding three different amphiphilic dyes. We found that BODIPY bearing one or three anchors failed in efficiently staining the PM: the derivative with one anchor showed low affinity to PM and exhibited strong fluorescence in water due to high solubility, whereas BODIPY with three anchors aggregated strongly in media and precipitated before binding to the PM. In sharp contrast, the BODIPY bearing two anchors (B-2AZ, MemBright-488) formed virtually nonfluorescent soluble aggregates in aqueous medium that quickly deaggregated in the presence of PM, leading to a bright soluble molecular form (quantum yield of 0.92). This fluorogenic response allowed for efficient probing of the PM at low concentration (20 nM) with high signal to background ratio images in mono- as well as two-photon excitation microscopy. B-2AZ proved to selectively stain the PM in a more homogeneous manner than the commercially available fluorescently labeled lectin WGA. Finally, it was successfully used in 3D-imaging to reveal fine intercellular tunneling nanotubes in KB cells and to stain the PM in glioblastoma cells in spheroids.

A facile two-photon fluorescent probe: an endoplasmic reticulum tracker monitoring ER stress and vesicular transport to lysosomes

The two-photon fluorescent organic probe ERLp selectively labels the endoplasmic reticulum in live cells and tumor spheroids. ERLp can also monitor ER dynamic changes during ER stress and vesicle transport from the ER to the lysosome in living cells.

Conjugated Polyimine Dynamers as Phase-Sensitive Membrane Probes

In this report, dynamic polyimines are introduced as multifunctional sensors of lipid bilayer phases. Under mildly acidic conditions, self-condensation of push-pull amino formyl fluorenes into polyimines occurs in solid- or liquid-ordered phases but not in liquid-disordered phases of vesicular membranes. The obtained conjugated polymers are characterized by a progressive red shift of the absorption maxima, the appearance of exciton-coupled circular dichroism (CD) bands, and fluorescence quenching. These characteristics allow multiple modes of detection of membrane phases, which are known to change under membrane tension.

A hydrophobic organelle probe based on aggregation-induced emission: Nanosuspension preparation and direct use for endoplasmic reticulum imaging in living cells

Organic fluorophores have a wide range of biological uses and are usually needed to be prepared as water-soluble compounds or nanoparticles for applications in aqueous biosystems owing to their hydrophobic properties, which often is a complex, time-consuming and high-cost process. Here, the nanoparticle preparation of hydrophobic fluorophores and their application in cell imaging have been investigated. It was found: a) fetal bovine serum (FBS) shows an excellent dispersion effect on hydrophobic small-molecule organic compounds; b) a hydrophobic C6-unsubstituted tetrahydropyrimidine (Me-THP-Naph) can be prepared as nanosuspensions utilizing cell culture medium with 10% FBS and directly be used as a specific real-time imaging probe for the endoplasmic reticulum (ER), a dynamic organelle playing a crucial role in many cellular processes. Compared with existing ER-targeted organic fluorescent probes, Me-THP-Naph, a product of an efficient five-component reaction that we developed, has unconventional aggregation-induced emission characteristics and shows advantages of low cost, long-term staining, good photostability, high signal-to-noise ratio and excellent biocompatibility, which make it a potential specific probe for real-time ER imaging. More importantly, this work affords a simple strategy for direct application of hydrophobic organic compounds in aqueous biological systems.

Assembly of BODIPY-carbazole dyes with liposomes to fabricate fluorescent nanoparticles for lysosomal bioimaging in living cells

A strategy was developed to change an ACQ organic dye into a fluorescent nanoparticle probe for lysosomal bioimaging.

Fluorescent Probes for Sensing and Imaging within Specific Cellular Organelles

Fluorescent probes have become powerful tools in biosensing and bioimaging because of their high sensitivity, specificity, fast response, and technical simplicity. In the last decades, researchers have made remarkable progress in developing fluorescent probes that respond to changes in microenvironments (e.g., pH, viscosity, and polarity) or quantities of biomolecules of interest (e.g., ions, reactive oxygen species, and enzymes). All of these analytes are specialized to carry out vital functions and are linked to serious disorders in distinct subcellular organelles. Each of these organelles plays a specific and indispensable role in cellular processes. For example, the nucleus regulates gene expression, mitochondria are responsible for aerobic metabolism, and lysosomes digest macromolecules for cell recycling. A certain organelle requires specific biological species and the appropriate microenvironment to perform its cellular functions, while breakdown of the homeostasis of biomolecules or microenvironmental mutations leads to organelle malfunctions, which further cause disorders or diseases. Fluorescent probes that can be targeted to both specific organelles and biochemicals/microenvironmental factors are capable of reporting localized bioinformation and are potentially useful for gaining insight into the contributions of analytes to both healthy and diseased states. In this Account, we review our recent work on the development of fluorescent probes for sensing and imaging within specific organelles. We present an overview of the design, photophysical properties, and biological applications of the probes, which can localize to mitochondria, lysosomes, the nucleus, the Golgi apparatus, and the endoplasmic reticulum. Although a diversity of organelle-specific fluorescent stains have been commercially available, our efforts place an emphasis on improvements in terms of low cytotoxicity, high photostability, near-infrared (NIR) emission, two-photon excitation, and long fluorescence lifetimes, which are crucial for long-time tracking of biological processes, tissue and body imaging with deep penetration and low autofluorescence, and time-resolved fluorescence imaging. Research on fluorescent probes with both analyte responsiveness and organelle targetability is a new and emerging area that has attracted increasing attention over the past few years. We have extended the diversity by developing organelle-specific responsive probes capable of detecting changes in biomolecular levels (reactive oxygen species, fluoride ion, hydrogen sulfide, zinc cation, thiol-containing amino acids, and cyclooxygenase-2) and the microenvironment (viscosity, polarity, and pH). Future research should give more considerations of the "low-concern" organelles, such as the Golgi apparatus, the endoplasmic reticulum, and ribosomes. In addition, given the tiny sizes of subcellular organelles (20-1000 nm), we anticipate that clearer visulization of the cellular events within specific organelles will rely on super-resolution optical microscopy with nanoscopic-scale resolution.

Solvatochromic fluorene-linked nucleoside and DNA as color-changing fluorescent probes for sensing interactions

A nucleoside bearing a solvatochromic push-pull fluorene fluorophore (dCFL ) was designed and synthesized by the Sonogashira coupling of alkyne-linked fluorene 8 with 5-iodo-2'-deoxycytidine. The fluorene building block 8 and labeled nucleoside dCFL exerted bright fluorescence with significant solvatochromic effect providing emission maxima ranging from 421 to 544 nm and high quantum yields even in highly polar solvents, including water. The solvatochromism of 8 was studied by DFT and ADC(2) calculations to show that, depending on the polarity of the solvent, emission either from the planar or the twisted conformation of the excited state can occur. The nucleoside was converted to its triphosphate variant dCFLTP which was found to be a good substrate for DNA polymerases suitable for the enzymatic synthesis of oligonucleotide or DNA probes by primer extension or PCR. The fluorene-linked DNA can be used as fluorescent probes for DNA-protein (p53) or DNA-lipid interactions, exerting significant color changes visible even to the naked eye. They also appear to be suitable for time-dependent fluorescence shift studies on DNA, yielding information on DNA hydration and dynamics.

Air-Stable Pd Catalytic Systems for Sequential One-Pot Synthesis of Challenging Unsymmetrical Aminoaromatics

The selective functionalization of dibromoaromatic scaffolds using air-stable palladium catalytic systems was carried out. This methodology involved rapid mono and diselective Buchwald-Hartwig aminations via microwave irradiation. The conditions were optimized to couple sequentially different moieties in one pot. Couplings with a wide scope of amines allowed accessing a new library of symmetrical and unsymmetrical derivatives (35 examples). Using this versatile method, a near-IR push-pull sensor was prepared installing the electron-donating and -withdrawing groups through a multicomponent reaction. These conditions revealed to be gram-scalable and adaptable to various groups; hence, promoting facile use in synthetic chemistry.

Rational Design of Push-Pull Fluorene Dyes: Synthesis and Structure-Photophysics Relationship

Our work surveyed experimental and theoretical investigations to construct highly emissive D-π-A (D=donor, A=acceptor) fluorenes. The synthetic routes were optimised to be concise and gram-scalable. The molecular design was first rationalised by varying the electron-withdrawing group from an aldehyde, ketotriazole or succinyl to methylenemalonitrile or benzothiadiazole. The electron-donating group was next varied from aliphatic or aromatic amines to saturated cyclic amines ranging from aziridine to azepane. Spectroscopic studies correlated with TD-DFT calculations provided the optimised structures. The selected push-pull dyes exhibited visible absorptions, significant brightness, important solvatofluorochromism, mega-Stokes shifts (>250 nm) and dramatic shifts in emission to the near-infrared. The current library includes the comprehensive characterization of 16 prospective dyes for fluorescence applications. Among them, several fluorene derivatives bearing different conjugation anchors were tested for coupling and demonstrated to preserve the photophysical responses once further bound.

Unusual Fluorescent Responses of Morpholine-functionalized Fluorescent Probes to pH via Manipulation of BODIPY's HOMO and LUMO Energy Orbitals for Intracellular pH Detection

Three uncommon morpholine-based fluorescent probes (A, B and C) for pH were prepared by introducing morpholine residues to BODIPY dyes at 4,4'- and 2,6-positions, respectively. In contrast to morpholine-based fluorescent probes for pH reported in literature, these fluorescent probes display high fluorescence in a basic condition while they exhibit very weak fluorescence in an acidic condition. The theoretical calculation confirmed that morpholine is unable to function as either an electron donor or an electron acceptor to quench the BODIPY fluorescence in the neutral and basic condition via photo-induced electron transfer (PET) mechanism because the LUMO energy of morpholine is higher than those of the BODIPY dyes while its HOMO energy is lower than those of the BODIPY dyes. However, the protonation of tertiary amines of the morpholine residues in an acidic environment leads to fluorescence quenching of the BODIPY dyes via d-PET mechanism. The fluorescence quenching is because the protonation effectively decreases the LUMO energy which locates between the HOMO and LUMO energies of the BODIPY dyes. Fluorescent probe C with deep-red emission has been successfully used to detect pH changes in mammalian cells.

Near-infrared fluorescent probes based on piperazine-functionalized BODIPY dyes for sensitive detection of lysosomal pH

Three acidotropic, near-infrared fluorescent probes based on piperazine-modified BODIPY dyes (A, B and C) have been developed for the sensitive and selective detection of lysosomal pH in living cells. Probes A and B display low solubilities in aqueous solutions, whereas probe C is highly water-soluble. The fluorescent responsive mechanism of these probes to lysosomal pH is based on intramolecular charge transfer (ICT) and potential photo-induced electron transfer from piperazine moieties at 3,5-positions to BODIPY cores in the near-infrared region. The sensitivity and selectivity of the probes to pH over metal ions have been investigated by spectroscopic analysis in aqueous solutions. The probes have low auto-fluorescence at physiological pH conditions, whereas their fluorescence intensities significantly increase when pH is shifted to an acidic condition. Furthermore, these three probes were successfully applied to the in vitro lysosome imaging inside normal endothelial and breast cancer cells.

Design and synthesis of fluorenone-based dyes: two-photon excited fluorescent probes for imaging of lysosomes and mitochondria in living cells

Three fluorenone-based two-photon fluorescent probes for specific targeting of lysosomes and mitochondria in cancer cells.

A novel pyrene-based two-photon active fluorescent dye efficiently excited and emitting in the 'tissue optical window (650-1100 nm)'

The development of two-photon (TP) active fluorophores remains an important issue. Dyes that can be excited and fluoresce efficiently in the 'tissue optical window' (650-1100 nm) are especially in demand to maximize the underlying performance of two-photon fluorescence microscopy (TPFM) as an advanced optical technique. Ideally, such dyes would be compatible with the 1050 nm femtosecond fibre laser, which has recently been developed as an inexpensive excitation source to make the TPFM technique universal. In this work, we designed and synthesized a novel pyrene-based acceptor-π-acceptor (A-π-A) dye, PY, which exhibited outstanding properties such as bright fluorescence (λ_em = 650 nm and Φ_FL = 0.80) and a large two-photon absorption cross-section (1100 GM (1 GM = 10^-50 cm⁴ per photon per molecule) at 950 nm and 380 GM at 1050 nm) in the tissue optical window. In living mitochondria, PY provided more sensitive microscopic images than current dyes and showed great potential to be a building block of TP active fluorescent probes for the 1050 nm fibre laser. We believe that the exceptional properties of PY will be extended to other fluorescent probes through further chemical modification.