I have a ASP.Net web handler that returns results of a query in JSON format

public static String dt2JSON(DataTable dt)
{
    String s = "{\"rows\":[";
    if (dt.Rows.Count > 0)
    {
        foreach (DataRow dr in dt.Rows)
        {
            s += "{";
            for (int i = 0; i < dr.Table.Columns.Count; i++)
            {
                s += "\"" + dr.Table.Columns[i].ToString() + "\":\"" + dr[i].ToString() + "\",";
            }
            s = s.Remove(s.Length - 1, 1);
            s += "},";
        }
        s = s.Remove(s.Length - 1, 1);
    }
    s += "]}";
    return s;
}

The problem is that sometimes the data returned has quotes in it and I would need to javascript-escape these so that it can be properly created into a js object. I need a way to find quotes in my data (quotes aren't there every time) and place a "/" character in front of them.

Example response text (wrong):

{"rows":[{"id":"ABC123","length":"5""},
{"id":"DEF456","length":"1.35""},
{"id":"HIJ789","length":"36.25""}]}

I would need to escape the " so my response should be:

{"rows":[{"id":"ABC123","length":"5\""},
{"id":"DEF456","length":"1.35\""},
{"id":"HIJ789","length":"36.25\""}]}

Also, I'm pretty new to C# (coding in general really) so if something else in my code looks silly let me know.