Specialty Dyes and Fluorescent Probes: A Sourcing Guide

Fluorescent probes sourcing is a specification exercise before it is a purchasing exercise. Most of the frustration buyers report with dyes traces back to a purchase order that named a compound and a quantity but not the parameters that decide whether the molecule works: where it absorbs, where it emits, how bright it is on the specific excitation source in the lab, how long it survives illumination, and which reactive handle it carries. A dye can arrive exactly as ordered and still be the wrong dye.

The difficulty is that fluorescent dyes are sold under conventions that do not match the rest of the fine chemical trade. A synthetic intermediate is quoted at 98 percent purity by HPLC and the number means what you expect. A cyanine dye is quoted at 80 or 90 percent dye content, and buyers routinely read that as a lower-grade material when it is nothing of the sort. Isomer content, counterion identity, residual moisture and the stated extinction coefficient all matter more to the experiment than the headline percentage does.

This guide is written for the person placing the order rather than the person running the microscope, though the two are often the same. It covers the photophysical parameters that belong in a specification, the major dye classes and what each is genuinely good at, the bioconjugation chemistry that constrains your choice of reactive derivative, storage and handling practice that determines whether the material survives to use, and the questions worth asking a supplier before you commit to a lot.

The Photophysics a Buyer Actually Needs

Six parameters describe a fluorescent dye well enough to buy it. Everything else is refinement.

Excitation and Emission Maxima

The excitation maximum is the wavelength at which the dye absorbs most efficiently; the emission maximum is where it radiates most of its recovered energy. Both are quoted in nanometers and both shift with solvent, pH and conjugation state — a dye measured in methanol will not report the same maxima in aqueous buffer, and a dye conjugated to a protein frequently red-shifts by several nanometers relative to the free form.

Match the excitation maximum to a laser line or LED you actually own. A dye peaking at 555 nm excited by a 561 nm laser is fine. The same dye on a 488 nm line will be excited at a small fraction of its peak absorbance, and no amount of detector gain will recover the signal-to-noise you gave away at that step. Ask for the spectra in a buffer resembling your assay conditions rather than the solvent that flatters the dye.

Stokes Shift and Why Large Is Better

Stokes shift is the separation between excitation and emission maxima. A dye with a 20 nm shift emits very close to where it absorbs, which forces a tight dichroic and a narrow emission filter and still lets some excitation light bleed into the detection channel. A dye with a 100 nm shift lets you use a generous emission band and collect far more of the emitted photons without collecting scattered excitation light along with them.

Large Stokes shift matters most in three situations: multiplexed panels where channels crowd each other, high-background samples where every stray photon costs contrast, and instruments with fixed filter sets you cannot change. It matters least in a well-separated two-color experiment on a fully configurable microscope. Cyanines characteristically show small shifts, typically 15 to 25 nm, because of their rigid polymethine structure. Coumarins and several engineered scaffolds do considerably better.

Quantum Yield, Extinction Coefficient, and Brightness

Quantum yield is the fraction of absorbed photons re-emitted as fluorescence, a dimensionless number between 0 and 1. Extinction coefficient is how strongly the dye absorbs at its maximum, quoted in units of M⁻¹cm⁻¹. Neither alone tells you how bright the dye will look down the eyepiece.

Brightness is the product of the two. A dye with an extinction coefficient of 250,000 M⁻¹cm⁻¹ and a quantum yield of 0.15 has a brightness figure of 37,500. A dye at 80,000 with a quantum yield of 0.45 sits at 36,000 — practically indistinguishable in use, despite one having three times the quantum yield of the other. Buyers who screen on quantum yield alone consistently pick the wrong dye.

Quantum yield is also strongly environment-dependent. Cyanines gain quantum yield when conjugation restricts their rotational freedom, and lose it in low-viscosity solvent. A quantum yield quoted without a solvent and a temperature is a marketing number, not a specification.

Photostability

Photobleaching is the irreversible photochemical destruction of the dye under illumination, usually by way of a triplet-state route involving dissolved oxygen. It sets a hard ceiling on how long you can image a field and how many frames a time-lapse can run.

There is no single photostability number that transfers across instruments, because bleaching rate scales with illumination intensity and depends on oxygen concentration, buffer composition and whether an antifade reagent is present. Treat vendor photostability claims as ordinal rather than absolute: rhodamines generally outlast fluoresceins, cyanine 5 generally outlasts cyanine 3 under matched conditions, and any dye run in a deoxygenated buffer with a triplet quencher outlasts the same dye in plain phosphate buffer. If photostability is critical, budget an afternoon to bleach-test two candidate dyes side by side on your own instrument. It is the only comparison that means anything.

Dye Classes and What Each Is For

Four families cover the large majority of research applications. Choosing the family first narrows the catalog far faster than browsing by wavelength.

Cyanines, Including the Near-Infrared Heptamethines

Cyanine dyes are polymethine chromophores built from two nitrogen-containing heterocyclic ends joined by a conjugated methine bridge. Bridge length sets the color: the trimethine cyanines (the Cy3 family) sit around 550 nm excitation, the pentamethines (the Cy5 family) around 650 nm, and the heptamethines push past 750 nm into the near-infrared.

Cyanines are the workhorses of labelling chemistry because they combine very high extinction coefficients — often 150,000 to 250,000 M⁻¹cm⁻¹ — with a synthetic scaffold that tolerates sulfonation for water solubility and functionalization for conjugation. Their weaknesses are a small Stokes shift, moderate quantum yield in free solution, and a tendency to stack and self-quench at high degrees of labelling.

Near-infrared heptamethine cyanines deserve separate treatment. In the roughly 700 to 900 nm window, hemoglobin absorbance has fallen away, water absorbance has not yet climbed, and endogenous tissue fluorophores such as flavins and collagen barely emit. The result is a spectral window where excitation light penetrates centimeters rather than micrometers and background autofluorescence is close to nil. That is why near-infrared dyes dominate whole-animal imaging, fluorescence-guided surgery research, and any application where the signal has to travel through tissue before it reaches a detector.

Two heptamethines in our stock illustrate the class. IR-775 chloride, dye content 90 percent (CAS 199444-11-6) is a chloro-substituted heptamethine carrying a central cyclohexenyl ring that rigidifies the polymethine chain and improves photostability relative to unbridged analogs; the meso-chloro position is also the usual handle for nucleophilic substitution if you want to append a targeting group. IR-820, dye content 80 percent (CAS 172616-80-7) is the sulfonated, water-soluble relative frequently used as a research alternative to indocyanine green, with better aqueous stability and a similar spectral position. Both sit within the broader sulfonyl compounds category by virtue of their sulfonate solubilizing groups.

Coumarins

Coumarins are benzopyranone scaffolds that fluoresce in the blue, typically exciting between 350 and 450 nm and emitting between 440 and 500 nm. They are small — often under 350 daltons — which matters when you are labelling something whose function a bulky dye would disturb.

Three properties make coumarins useful out of proportion to their raw brightness. First, the Stokes shift is generous, commonly 50 to 100 nm, which makes them forgiving on filter selection. Second, the 7-position hydroxy and amino coumarins are excellent enzyme substrate cores: fluorescence of the free coumarin is strongly quenched when the 7-position is acylated or glycosylated, so enzymatic cleavage produces a large turn-on signal against a dark background. Umbelliferone and aminomethylcoumarin substrates for glycosidases, phosphatases and proteases all exploit this. Third, coumarins pair well as FRET donors to green and yellow acceptors, because their emission band overlaps fluorescein-class absorbance neatly.

Coumarin 343, dye content 98 percent (CAS 55804-65-4) is the classic laser-grade member of this family: a julolidine-fused coumarin carrying a free carboxylic acid, with high quantum yield and a rigidified amino donor that suppresses the twisted intramolecular charge-transfer decay pathway limiting simpler 7-aminocoumarins. The free acid makes it directly usable in carbodiimide couplings. Its fused nitrogen-heterocyclic core places it in the heterocyclic compounds category alongside the other heterocyclic scaffolds we stock.

For labelling work without an in-house activation step, Coumarin 343 X NHS ester (CAS 946123-12-2) is the pre-activated form, with an aminohexanoyl spacer between the chromophore and the reactive ester. The spacer is not cosmetic: it moves the dye away from the biomolecule surface, which reduces both steric interference with binding and the environmental quenching that a chromophore pressed against a protein backbone often suffers.

Metallochromic and Colorimetric Indicators

Not every probe is fluorescent. Metallochromic indicators change absorbance rather than emission on metal binding, and for certain ion measurements they remain the more robust choice.

Arsenazo III calcium-sensitive dye (CAS 1668-00-4) is the standard example. It is a bis-azo derivative of chromotropic acid that forms a strongly colored complex with calcium, shifting absorbance in a way that supports ratiometric measurement at paired wavelengths. Because the readout is absorbance, it is unaffected by the photobleaching and dye-loading variability that complicate fluorescent calcium indicators, and it works on a plate reader or a spectrophotometer without any fluorescence optics at all. It is widely used for calcium transients in permeabilized preparations, for calcium content assays on isolated organelles, and as an indicator in analytical titrations. It also responds to other divalent and lanthanide ions, so a chelator control belongs in the experimental design.

Xanthenes, for Orientation

The xanthene family — fluoresceins and rhodamines — is where most people first meet fluorescence, and it is worth knowing where they sit even if you buy elsewhere. Fluoresceins excite near 490 nm and emit near 520 nm, are exceptionally bright, and have two well-known problems: fluorescence is strongly pH-dependent below pH 7, and they photobleach quickly. Rhodamines excite roughly 540 to 580 nm, are pH-insensitive across the physiological range, and are markedly more photostable, at the cost of a smaller Stokes shift and greater hydrophobicity. If you are replacing a fluorescein-based assay that suffers from bleaching or pH drift, a rhodamine or a bright cyanine is the usual substitution.

Reactive Derivatives and Bioconjugation

If you are attaching the dye to something, the reactive group determines the chemistry, the pH and the site selectivity. Choose it before you choose the color.

NHS Esters for Amines

N-hydroxysuccinimidyl esters acylate primary amines, which on a protein means lysine side chains and the alpha-amino group of the N-terminus. The reaction is run in aqueous buffer at pH 8.0 to 8.5. Below pH 7.5 the lysine epsilon-amine is largely protonated and unreactive; above pH 9 competing hydrolysis of the NHS ester accelerates enough to waste most of the reagent before it finds a target.

Practical constraints follow from that pH window. Buffers containing free amines — Tris, glycine, ammonium salts — consume the reagent and must be exchanged out before labelling. The NHS ester itself is moisture-sensitive: dissolve it in anhydrous DMSO immediately before use and add the organic stock to the aqueous protein solution, keeping final DMSO below about 10 percent by volume for most proteins.

Amine labelling is the easy path, because a typical protein presents many accessible lysines. That abundance is also its weakness. Labelling is stochastic across those sites, so you obtain a heterogeneous population, and if one of the reactive lysines sits in the active site or the binding interface you lose activity in an unpredictable fraction of the material.

Maleimides for Thiols

Maleimides undergo Michael addition with free thiols, which on a protein means reduced cysteine. The working pH window is 6.5 to 7.5. Selectivity is the whole point: within that window maleimide reacts with thiol several orders of magnitude faster than with amine. Push above pH 8 and two things go wrong at once — amine cross-reactivity rises and the maleimide ring hydrolyzes to an unreactive maleamic acid.

Thiol labelling gives far better site control, since free cysteines are rare and can be engineered in at a chosen position. The cost is preparation: most cysteines in a secreted or purified protein are tied up in disulfides and must be reduced first, typically with TCEP, which unlike DTT does not need removing before the maleimide step because it carries no competing thiol of its own.

Degree of Labelling and Why More Is Not Better

Degree of labelling, or DOL, is the average number of dye molecules per biomolecule. It is measured spectrophotometrically after purifying away free dye:

1. Read absorbance at 280 nm and at the dye’s absorbance maximum.
2. Correct the 280 nm reading for the dye’s own absorbance there, using the supplier’s correction factor.
3. Divide the corrected 280 nm value by the protein extinction coefficient to get protein molarity.
4. Divide the dye maximum absorbance by the dye extinction coefficient to get dye molarity.
5. Divide dye molarity by protein molarity.

Step 4 is where a missing extinction coefficient stops the calculation dead. This is the practical reason to insist on a stated, material-relevant extinction coefficient at the quoted maximum rather than a literature value copied out of a review.

Over-labelling quenches signal rather than amplifying it. Beyond roughly 4 to 8 dyes per antibody for most cyanines, adjacent chromophores on the same molecule stack, form non-fluorescent H-aggregates, and self-quench through energy transfer into those dark states. Total fluorescence per antibody peaks and then falls. Heavily labelled conjugates also lose solubility and, in the case of antibodies, frequently lose affinity. A DOL in the 2 to 6 range suits most immunofluorescence and flow work; the optimum is dye-specific and worth determining once for a reagent you will use repeatedly.

Purity, Dye Content, and What a Specification Should Say

Dye content is the convention for reporting fluorescent dye assay, and it means something different from chromatographic purity. Understanding the difference prevents most specification disputes before they start.

What Dye Content Measures

Dye content is the mass fraction of the shipped solid that is the chromophore. The remainder is counterions, residual solvent from the final crystallization or lyophilization, and adsorbed water. Many dyes are salts — sulfonates, iodides, chlorides, perchlorates — and many are hygroscopic enough to pick up several percent moisture from room air during weighing.

So a lot quoted at 80 percent dye content is not a lot that is 20 percent contaminated with related substances. It is a lot in which 80 percent of the weight is the dye and 20 percent is counterion and water. Chromatographic purity on the dye fraction can simultaneously be 98 percent or better. The two figures measure different things, and a good certificate of analysis reports both.

The practical consequence appears at the balance. If you need 1.0 mg of active dye and the lot is 80 percent dye content, weigh 1.25 mg. Buyers who ignore this run consistently under-concentrated stocks and blame the dye. Better still, determine your working stock concentration spectrophotometrically using the extinction coefficient rather than gravimetrically — that measurement is immune to both dye content and weighing error on small masses.

Parameter	What it measures	Typical convention	Why it matters to the buyer
Dye content	Mass fraction of solid that is chromophore	80 to 98 percent	Sets the weighing correction for stock preparation
Chromatographic purity	Percent of dye-related material that is the target	95 to 99 percent by HPLC	Bounds interference from related colored impurities
Isomer content	Ratio of regio- or geometric isomers	Reported for functionalized cyanines	Affects spectral shape and conjugation reproducibility
Extinction coefficient	Absorptivity at a stated maximum	M⁻¹cm⁻¹ with solvent stated	Required for the DOL calculation and stock quantitation
Water content	Adsorbed and bound moisture	Karl Fischer, percent w/w	Explains the dye content shortfall and predicts hygroscopicity

Isomer Content in Cyanines

Unsymmetrical cyanines and functionalized derivatives can be produced as mixtures of positional isomers — commonly the 5- and 6-substituted regioisomers of a carboxy-functionalized dye. The isomers are spectrally near-identical, which is why mixed lots are routinely sold and routinely acceptable. They are not identical in conjugation kinetics or in chromatographic behavior, so if you are developing a method that has to reproduce lot to lot, or you are running LC-MS on the conjugate and need a single clean peak, specify a single isomer and expect to pay for the separation. For most imaging and flow applications the mixture is fine.

The Extinction Coefficient Is Not Optional

Insist on a stated extinction coefficient with the solvent and the wavelength at which it was determined. Without it you cannot compute a DOL, cannot standardize a stock concentration, and cannot compare brightness across candidates. Published values for the same nominal compound vary by more than you would expect, largely because of solvent and aggregation differences, so a supplier figure tied to their material is worth more than a citation. PubChem is a reasonable cross-check on structure, CAS assignment and identifiers, and the NIST Chemistry WebBook is the reference source for spectroscopic standards where a traceable comparison is needed.

Handling and Storage

Fluorescent dyes fail in storage more often than they fail in chemistry. The failure modes are light, moisture and freeze-thaw, in roughly that order of frequency.

Light Protection and Desiccation

Ship and store dyes in amber vials or foil-wrapped clear vials. The exposure that matters is cumulative, so a vial left on a bench under room lighting between weighings degrades measurably over months even though no single exposure looks significant. Work under reduced light for the near-infrared cyanines in particular, which are among the most photolabile classes in common use.

Store the solid desiccated. Most dyes are hygroscopic, and adsorbed water does two damaging things: it inflates your weighing error against the dye content figure, and for reactive derivatives it slowly hydrolyzes the active group. A reactive NHS ester left in a poorly sealed vial at room temperature can lose most of its conjugation efficiency in a matter of weeks while looking entirely unchanged.

Stock Solutions and Aliquoting

Prepare concentrated stocks in anhydrous DMSO, typically 1 to 10 mM. DMSO is chosen because it dissolves nearly every dye class, is miscible with aqueous buffer, and is tolerated by most proteins at low final percentages. Use a fresh or freshly-dried bottle — DMSO is itself hygroscopic, and an old open bottle can carry several percent water, which defeats the purpose entirely for a reactive ester.

Aliquot into single-use volumes immediately and freeze. Repeated freeze-thaw is the single most common avoidable cause of a reactive dye underperforming: each cycle admits condensation into the vial, and that water hydrolyzes the reactive group. For non-reactive dyes freeze-thaw is far less damaging, though aggregation on repeated cycling can still shift the spectrum enough to matter in a quantitative assay.

Typical Stability Expectations

Stated shelf lives vary by supplier and by compound, and it is fair to ask for the basis of the number rather than assume it. As general expectations under correct storage:

• Non-reactive solids, desiccated, minus 20 degrees Celsius, dark: commonly quoted at one to several years.
• Reactive NHS esters and maleimides, desiccated, minus 20 degrees Celsius: shorter, and most sensitive to moisture exposure during handling. Warm the vial to room temperature before opening so condensation forms outside rather than inside.
• DMSO stocks, single-use aliquots, minus 20 degrees Celsius, dark: months for non-reactive dyes, considerably less for reactive esters, which many labs simply prepare fresh.
• Aqueous working solutions: treat as same-day for anything reactive, and confirm the spectrum before use for anything held longer.

Where a supplier’s stability claim matters to your process, ask whether it comes from real-time data or from an accelerated study, and confirm with your supplier rather than assuming. Our analytical services group runs the HPLC, LC-MS and Karl Fischer work that underpins that kind of determination.

Applications Map: Matching Dye to Experiment

The application dictates which parameter you optimize. This is the fastest route from a vague requirement to a shortlist.

Application	Optimize for	Typically avoid	Notes
Flow cytometry	Brightness at a fixed laser line, narrow emission	Broad-emission dyes in crowded panels	Panel design is constrained by the instrument’s fixed filters more than by the dye catalog
Fluorescence microscopy	Photostability first, then brightness	Fluoresceins for long time-lapse	Bleach-test candidates on your own instrument before committing
In vivo and deep-tissue imaging	Near-infrared emission, 700 to 900 nm	Anything below about 650 nm	Autofluorescence and tissue absorbance dominate the decision
FRET assays	Donor emission overlapping acceptor absorbance, large donor Stokes shift	Pairs with heavy direct-excitation crosstalk	Coumarin donors are the classic pairing for green acceptors
High-throughput screening	Signal window and stability across the read period	Dyes needing rigorous oxygen exclusion	Plate-reader assays reward robustness over peak brightness
Calcium and ion measurement	Ratiometric readout, dynamic range across the physiological span	Single-wavelength probes where loading varies	Absorbance indicators such as Arsenazo III avoid loading artifacts entirely

How Do I Pick Between Two Dyes With Similar Spectra?

Compare, in this order: brightness (extinction coefficient multiplied by quantum yield, measured in a solvent resembling your assay), photostability under your illumination, then aqueous solubility and aggregation tendency at the concentration you will actually use. Price sits below all three. A cheaper dye that forces you to double illumination intensity costs you in bleaching and phototoxicity, and that cost shows up in your data rather than on the invoice.

Where Does Combinatorial Chemistry Meet Dye Work?

Fluorescence readouts are the backbone of library screening, so the probe choice determines the assay’s signal window and the two decisions should be made together rather than in sequence. Our companion posts on combinatorial chemistry libraries in drug discovery and on building block libraries: buy, build or commission cover the library side of that decision, including how compound-set design interacts with assay format.

Sourcing: What to Ask For and When to Go Custom

Ask for four documents before you buy, and be specific about which of them must be lot-specific.

The Four-Document Request

1. Absorbance and emission spectra, with the solvent stated. Ideally in a buffer close to your assay conditions, not only in methanol.
2. Extinction coefficient at a stated wavelength and solvent. Required for stock quantitation and for the DOL calculation.
3. Dye content percentage, plus chromatographic purity as a separate figure and, for functionalized cyanines, isomer content.
4. A lot-specific certificate of analysis. Generic catalog data is a specification; a COA is evidence about the material in the vial you are actually receiving.

The distinction in point four is the one worth defending. Structure confirmation by NMR or LC-MS and a water determination by Karl Fischer are reasonable additions for any dye entering a regulated or method-validated workflow. Our guides to analytical testing methods for pharma buyers and reference standard sourcing and certification cover the documentation hierarchy in more depth, and the ACS journals remain the best primary literature source for photophysical characterization methodology.

When Catalog Material Is Not Enough

Three situations push you toward custom work:

You need a conjugate rather than a dye. Labelling a protein, an oligonucleotide or a small molecule is a discrete chemistry project with a purification and a characterization attached. If your lab does not have the purification capability — size exclusion or desalting for proteins, reverse-phase for oligonucleotides — outsourcing the conjugation is usually cheaper than building that capability for one program.

You need a derivative that is not sold. A different reactive handle on a known chromophore, a different spacer length, a specific isomer, a clickable azide or alkyne version, a differently-sulfonated analog for solubility. These are ordinary custom synthesis projects. The chromophore chemistry is established; the work sits in the functionalization and the purification.

You need scale the catalog does not offer. Research dyes are usually sold in 10 to 100 mg units. An assay moving into routine screening, or a preclinical imaging program, can need grams. Scaling a dye is not merely running the same route bigger — chromatographic purification that is trivial at 50 mg becomes the rate-limiting step at 50 g, and the crystallization or controlled precipitation that has to replace it needs developing. That is a process chemistry task; our note on scale-up challenges in custom synthesis covers the general shape of it.

Domestic Sourcing and Lead Time

Specialty dyes are a concentrated market, and many individual chromophores have only a handful of producers worldwide. For research quantities that is usually tolerable. For anything sitting on a program timeline it is worth knowing where your material is made and whether a second route exists. Our US-based custom synthesis capability map sets out what can realistically be made domestically and at what scale, and academic groups working through this for the first time may find our academic research page a useful starting point. The full stocked catalog, dyes included, is browsable from our products index.

ChemContract Research stocks the probes described here and runs the chemistry behind them: custom synthesis from milligram to multi-ton for novel chromophores, reactive derivatives and conjugates; analytical services including HPLC, LC-MS, NMR and Karl Fischer for lot characterization and spectral verification; and combinatorial and automated synthesis for probe-set generation and structure-property work on dye scaffolds. Tell us the instrument configuration and the labelling target, send us the specification, and we will return a recommendation and a quote within 24 hours.

Frequently Asked Questions

What is Stokes shift and why does it matter when buying a dye?

Stokes shift is the gap in nanometers between a dye’s excitation maximum and its emission maximum. A large shift makes it easier to separate emitted light from scattered excitation light, which means simpler filter selection, less bleed-through, and a better signal-to-background ratio. Cyanines typically show shifts of 15 to 25 nm; some coumarins and engineered probes exceed 100 nm.

What does dye content 90 percent actually mean?

Dye content is the mass fraction of the shipped solid that is the chromophore itself. The balance is counterions, residual solvent and adsorbed moisture, which dyes hold tenaciously because many are hygroscopic salts. It is not the same as chromatographic purity. A lot can be 90 percent dye content and still better than 98 percent pure by HPLC on the dye fraction.

How is dye brightness calculated?

Brightness is the product of the molar extinction coefficient at the excitation maximum and the fluorescence quantum yield. A dye with an extinction coefficient of 250,000 and a quantum yield of 0.2 is brighter than one at 80,000 with a yield of 0.5. Compare brightness rather than quantum yield alone, since quantum yield in isolation is misleading.

When should I use an NHS ester versus a maleimide?

Use an NHS ester to label primary amines, meaning lysine side chains and the protein N-terminus, at pH 8.0 to 8.5. Use a maleimide to label free thiols on cysteine, at pH 6.5 to 7.5 where maleimide selectivity for thiol over amine is highest. Amine labelling is easier but less site-specific; thiol labelling is more controlled but usually needs a reduction step first.

Why do near-infrared dyes dominate in vivo imaging?

In the roughly 700 to 900 nm window, tissue absorbance from hemoglobin and water is at a local minimum and endogenous autofluorescence is low, so photons travel further and background is lower. Heptamethine cyanines such as IR-775 and IR-820 sit in this window, which is why they are the workhorses for whole-animal and deep-tissue fluorescence imaging.

How should fluorescent dyes be stored?

Store the solid desiccated, protected from light, at minus 20 degrees Celsius for most dyes and minus 20 or colder for reactive esters. Prepare concentrated stocks in anhydrous DMSO, aliquot into single-use volumes, and freeze. Repeated freeze-thaw and atmospheric moisture are the two most common causes of a reactive dye losing conjugation efficiency before its stated shelf life.