As a biological stain, Toluidine Blue does not have a single molecular target but rather binds electrostatically to negatively charged (anionic) molecules in tissues. It has a high affinity for acidic tissue components such as the sulfate and phosphate groups found in nucleic acids (DNA/RNA) and proteoglycans. The staining mechanism is based on its chemical structure as a cationic dye, which allows it to bind to these polyanionic molecules. This property makes it a valuable tool for visualizing structures such as nuclei (blue), mast cell granules (purple/red), cartilage matrix (purple), and plant cell walls (blue-green).

Guidelines (This is our suggested protocol; it should be adjusted to meet your unique needs; it is merely a guideline). Identification of Mast cell infiltration[1]: 1. First, tissue samples were immersed in 10% buffered formalin for a full day. 2) Take the tissues out of the fixative and submerge them in 70% ethanol. 3) Embed in paraffin: Place tissues in paraffin blocks after progressively incubating them for 30 minutes in each of the following:70,85,95, and 100% ethanol, xylene, and melted paraffin. 4) Slice tissue fixed in paraffin, measuring 5-7 μm, and transfer the pieces onto glass slides. 5) Before using, bake the slides at 55°C for at least one hour. 2. Tissue staining: 1) Remove wax from paraffin-embedded tissue sections by immersing slides in xylene (or Histo-Clear) for five minutes. 2) Rehydrate tissue by immersing it in 100%, 95%, and 70% ethanol for five minutes each, followed by a two-minute immersion in distilled water. 2) Submerge the slides for 70 seconds in Harris' hematoxylin solution. 3) Rinse the slides in clean tap water two or three times to quickly remove any remaining hematoxylin. Tap the slides on paper towels for a few seconds to get rid of extra water. 4) Quickly submerge slides in 70% alcohol three times, then repeat the washing process for three minutes under running tap water. Take out the extra water. 5) Submerge slides for a maximum of one minute in Scott's bluing solution. In the nucleus, hematoxylin is gradually changed by this process into an insoluble blue color. 6) Check that the nuclei are correctly stained by looking at the slides using a strong light microscope. 7) Give slides a three-minute wash in fresh tap water, then drain any extra. 8) Submerge slides for four minutes in a clean, about 1.1% Toluidine Blue purity 36% solution (Note 1). 9) Give slides a three-minute wash in fresh tap water, then drain any extra. 10) Dip the slides quickly three times in a batch of 70% ethanol, and then five times in a solution of 5% eosin. 11) To dehydrate the slides, immerse them in 70% (for two minutes), 95% (for two minutes each), and 100% ethanol (for two minutes each). To finish, submerge the slides in xylene (or Histo-Clear) for two minutes each. 12) Use mounting medium with a xylene base to permanently mount each slide with a sanitized coverslip. 13) Prior to examining slides under a bright-light microscope, allow them to dry for at least 60 minutes. First, make a 1.1% solution of Toluidine Blue purity 36% by mixing 1.1 g of the pure Toluidine Blue with 100 mL of 0.1 M sodium acetate buffer pH 4. Mix thoroughly. To bring the pH level down to 2.0–2.5, add a few drops of 1 M hydrochloric acid. Staining is done in a light-free environment at 25 °C. All set for use. Utilizing 36% pure Toluidine Blue [2][3]: 1. mucins found in connective tissue, particularly acid mucins. The background is tinted blue, while the tissue stains purple to red. 2. Heparin and histamine cause the granules of mast cells to tint purple. 3. Although amyloid stain is blue, polarized light causes it to exhibit a brilliant red birefringence. 4. Granules of endocrine cells are stained purple to crimson (stain concentration: 0.01%). 5. Sulfatides have a reddish-brown or yellow stain. Lipids are only stained if they are sufficiently acidic to cause a metachromatic shift. 6. Granules of polymerized inorganic polyphosphate, which stain red violet, are seen in Corneybacterium diphtheria. 7. Helicobacter exhibits a dark blue staining over a background that varies in color (1% stain concentration). 8. Frozen sections can be stained with Toluidine Blue purity 36% due to the staining process's speed (10–20 s) and improved cell clarity. 9. The lignins of the plant, the phloem, and the remaining portion of the plant can all be dyed blue-green or pale blue-green using toluidine blue purity 36%.

---

In vitro, Toluidine Blue is not used in traditional "activity" assays but rather in staining protocols for cultured cells or histological sections. For example, in cell biology, it is used as a simple and rapid stain to assess cell density and morphology in 96-well plates. In plant biology, it can be used to stain xylem, phloem, and lignified tissue. It can also be used to stain frozen sections of tissue to visualize general morphology or to identify specific features like mitotic figures, mast cells, or cartilage. The characteristic metachromatic shift (from blue to purple/red) upon binding to certain proteoglycans is a key readout of its activity.

In vivo, Toluidine Blue O is not generally used as a systemic therapeutic agent but can be used as a diagnostic tool. It is used in vivo in medical and dental procedures to identify potentially cancerous lesions (vital staining). When applied topically to suspicious tissues, the dye is taken up by rapidly dividing cells, staining them a darker blue, which helps guide biopsy. It is also used in ophthalmology for certain diagnostic procedures. The in vivo activity is based on the same principle of its affinity for acidic tissue components.

Toluidine Blue is not typically used in enzyme/receptor binding assays, as it is a dye for staining and visualizing structures. However, for quality control or standardization, the binding of the dye to a standard substrate can be studied. A simple protocol involves preparing a solution of Toluidine Blue (0.1% w/v) in a buffer (e.g., 0.1 M acetate buffer, pH 4.5). This solution is then added to a microplate containing a standard amount of a polyanionic molecule like heparin or a section of cartilage. After a short incubation (5-10 minutes), the plate is washed, and the bound dye is eluted using a solvent (e.g., 50% ethanol containing 0.1 M HCl). The absorbance of the eluted dye is then measured at its characteristic wavelength (around 630 nm) to quantify the binding.

For a standard in vitro cell staining protocol with cultured cells, the medium is first removed, and the cells are washed with phosphate-buffered saline (PBS). The cells are then fixed with a fixative like 10% neutral buffered formalin for 10-15 minutes, followed by washing with PBS. The cells are then stained with a 0.1% solution of Toluidine Blue in distilled water for 1-5 minutes. After staining, the cells are gently rinsed with distilled water to remove excess stain. The stained cells are then allowed to air-dry. The nuclei and any metachromatic granules within the cells can be readily visualized under a light microscope.

A common in vivo protocol for Toluidine Blue is its use as an intravital dye for lesion identification in oral tissues. The area of interest is first cleaned and dried. A 1% aqueous solution of Toluidine Blue is applied topically to the tissue using a cotton swab or by rinsing for 30-60 seconds. The area is then gently rinsed with water. Any remaining blue-stained areas are visually examined and recorded, indicating regions of high nucleic acid content that are suspicious for dysplasia or malignancy. This simple procedure helps guide clinicians for biopsy.

Toluidine Blue is a small, cationic dye molecule. When applied topically for diagnostic staining, it is not typically administered systemically; therefore, traditional pharmacokinetic (absorption, distribution, metabolism, excretion) parameters are not routinely measured. Its behavior is characterized by its rapid binding to tissue components at the application site. The 36% purity is a standardized formulation that ensures batch-to-batch consistency of staining intensity, which is the primary "quality control" for this compound. Its distribution is limited to the site of application, and it is easily washed away from unbound tissues.

Toluidine Blue O has a long history of safe use as a laboratory reagent and diagnostic tool. However, it is not intended for systemic administration. In experimental settings, high systemic doses of phenothiazine dyes have been associated with methemoglobinemia and hemolytic anemia. As a topical agent, it is generally well-tolerated, with occasional mild local irritation at the application site. It is also known to be a mild photosensitizer. Standard laboratory safety precautions should be followed, including the use of a lab coat and gloves, as it can stain skin and clothing. It is a research chemical, not for human use.

[1]. Detection of Infiltrating Mast Cells Using a Modified Toluidine Blue Staining. Methods Mol Biol. 2017;1627:213-222.

[2]. Toluidine blue: A review of its chemistry and clinical utility. J Oral Maxillofac Pathol. 2012 May;16(2):251-5.

[3]. Polychromatic staining of plant cell walls by toluidine blue O. Protoplasma, 1964, 59(2): 368-373.

Toluidine Blue O is a metachromatic dye useful for staining histological sections, frozen sections, and cultured cells for microscopic analysis. Its ability to differentially stain nuclei (blue) and polysaccharides, mast cell granules, and cartilage (purple/red) makes it a routine histological stain for identifying these structures. The 36% purity is a standardized form of the dye, with a manufacturer's purity of 99.34%. Toluidine Blue is also known as Tolonium chloride. It is commonly used in basic histology, pathology, and developmental biology laboratories. It is not a therapeutic drug, is not in clinical trials, and is not approved for human use.

Toluidine Blue;92-31-9

Solid powder

2934.99.9001

Powder      -20°C    3 years

                     4°C     2 years

In solvent   -80°C    6 months

                  -20°C    1 month

Note: (1). This product requires protection from light (avoid light exposure) during transportation and storage.  (2). Please store this product in a sealed and protected environment (e.g. under nitrogen), avoid exposure to moisture.

Room temperature (This product is stable at ambient temperature for a few days during ordinary shipping and time spent in Customs)

May dissolve in DMSO (in most cases), if not, try other solvents such as H2O, Ethanol, or DMF with a minute amount of products to avoid loss of samples

Note: Listed below are some common formulations that may be used to formulate products with low water solubility (e.g. < 1 mg/mL), you may test these formulations using a minute amount of products to avoid loss of samples.

---

Injection Formulation 1: DMSO : Tween 80： Saline = 10 : 5 : 85 (i.e. 100 μL DMSO stock solution → 50 μL Tween 80 → 850 μL Saline)
*Preparation of saline: Dissolve 0.9 g of sodium chloride in 100 mL ddH ₂ O to obtain a clear solution.
Injection Formulation 2: DMSO : PEG300 ：Tween 80 : Saline = 10 : 40 : 5 : 45 (i.e. 100 μL DMSO → 400 μLPEG300 → 50 μL Tween 80 → 450 μL Saline)
Injection Formulation 3: DMSO : Corn oil = 10 : 90 (i.e. 100 μL DMSO → 900 μL Corn oil)
Example: Take the Injection Formulation 3 (DMSO : Corn oil = 10 : 90) as an example, if 1 mL of 2.5 mg/mL working solution is to be prepared, you can take 100 μL 25 mg/mL DMSO stock solution and add to 900 μL corn oil, mix well to obtain a clear or suspension solution (2.5 mg/mL, ready for use in animals).

View More

Injection Formulation 4: DMSO : 20% SBE-β-CD in saline = 10 : 90 [i.e. 100 μL DMSO → 900 μL (20% SBE-β-CD in saline)]
*Preparation of 20% SBE-β-CD in Saline (4°C，1 week): Dissolve 2 g SBE-β-CD in 10 mL saline to obtain a clear solution.
Injection Formulation 5: 2-Hydroxypropyl-β-cyclodextrin : Saline = 50 : 50 (i.e. 500 μL 2-Hydroxypropyl-β-cyclodextrin → 500 μL Saline)
Injection Formulation 6: DMSO : PEG300 : castor oil : Saline = 5 : 10 : 20 : 65 (i.e. 50 μL DMSO → 100 μLPEG300 → 200 μL castor oil → 650 μL Saline)
Injection Formulation 7: Ethanol : Cremophor : Saline = 10： 10 : 80 (i.e. 100 μL Ethanol → 100 μL Cremophor → 800 μL Saline)
Injection Formulation 8: Dissolve in Cremophor/Ethanol (50 : 50), then diluted by Saline
Injection Formulation 9: EtOH : Corn oil = 10 : 90 (i.e. 100 μL EtOH → 900 μL Corn oil)
Injection Formulation 10: EtOH : PEG300：Tween 80 : Saline = 10 : 40 : 5 : 45 (i.e. 100 μL EtOH → 400 μLPEG300 → 50 μL Tween 80 → 450 μL Saline)

---

Oral Formulation 1: Suspend in 0.5% CMC Na (carboxymethylcellulose sodium)
Oral Formulation 2: Suspend in 0.5% Carboxymethyl cellulose
Example: Take the Oral Formulation 1 (Suspend in 0.5% CMC Na) as an example, if 100 mL of 2.5 mg/mL working solution is to be prepared, you can first prepare 0.5% CMC Na solution by measuring 0.5 g CMC Na and dissolve it in 100 mL ddH2O to obtain a clear solution; then add 250 mg of the product to 100 mL 0.5% CMC Na solution, to make the suspension solution (2.5 mg/mL, ready for use in animals).

View More

Oral Formulation 3: Dissolved in PEG400
Oral Formulation 4: Suspend in 0.2% Carboxymethyl cellulose
Oral Formulation 5: Dissolve in 0.25% Tween 80 and 0.5% Carboxymethyl cellulose
Oral Formulation 6: Mixing with food powders

---

Note: Please be aware that the above formulations are for reference only. InvivoChem strongly recommends customers to read literature methods/protocols carefully before determining which formulation you should use for in vivo studies, as different compounds have different solubility properties and have to be formulated differently.

---

 (Please use freshly prepared in vivo formulations for optimal results.)